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AN  OUTLINE  OF  PSYCHOBIOLOGY 


BY 

KNIGHT  DUNLAP 

ASSOCIATE   PROFESSOR   OF   PSYCHOLOGY 
IN    THE    JOHNS     HOPKINS    UNIVERSITY 


BALTIMORE 

THE  JOHNS  HOPKINS  PRESS 

J914 


/s 


TO 
GEORGE  M.  STRATTON 


PREFACE. 

This  outline  is  intended  to  aid  those  students  of  psychology  who  have 
had  no  courses  in  biology  covering  the  morphological  and  physiological 
data  which  are  directly  contributory  to  psychology.  It  is  designed  to  con- 
vey the  elementary  information  which  is  absolutely  necessary,  and  to  stimu- 
late the  student  to  further  reading.  Since  the  time  which  a  psychologist 
can  give  to  the  study  of  biology  is  narrowly  limited,  it  is  essential  that 
strong  emphasis  should  be  placed  on  such  details  as  are  of  the  greatest 
psychological  significance,  although  this  results  in  a  treatment  which,  from 
the  physiological  point  of  view,  is  extremely  unbalanced. 

Heretofore,  psychologists  who  have  recognized  the  value  of  physiology 
have  confined  their  attention  almost  exclusively  to  neurology.  This  neur- 
ology has  been  of  little  use  to  the  psychologist,  except  as  a  terminological 
scheme  in  which  he  could  restate  his  psychological  facts  and  speculations. 
Of  late  it  has  been  becoming  clear  that  the  pressing  need  in  psycho-physi- 
ology is  for  the  study  of  muscle  and  gland,  and  that  only  through  the 
study  of  these  tissues  in  their  structural  and  functional  relation  to  nervous 
tissue  can  neurology  be  made  psychologically  valuable.  It  is  this  point  of 
view  which  has  dominated  the  preparation  of  this  outline. 

I  hope  that  this  book,  which  was  prepared  primarily  for  the  use  of  my 
own  classes,  may  be  of  service  to  other  psychologists,  at  least  until  a  more 
systematic  and  comprehensive  text  becomes  available.  Since  it  is*  at  the 
time  of  writing,  the  first  book  of  its  kind,  it  is  entitled  to  the  credit,  and 
also  to  the  leniency  usually  extended  to  pioneers. 

The  cuts  which  accompany  the  text  are  from  various  sources.  Some  are 
familiar  from  having  appeared  in  many  places.  I  am  much  indebted  to 
those  who  have  kindly  given  me  permission  to  use  or  reproduce  their  illus- 
trations, especially  to  Dr.  Ramon  y  Cajal.  and  to  the  authors,  editors, 
and  publishers  of  Bailey's  Histology  and  Cunningham's  Anatomy  (William 
Wood  and  Co.,  New  York)  ;  Lewis  and  Stohr's  Histology  (P.  Blakis- 
ton's  Son  &  Co.,  Philadelphia)  ;  Quain's  Anatomy  (Eleventh  Edition, 
Longmans,  Green  and  Co.,  New  York)  ;  Barker's  The  Nervous  System 
(D.  Appleton  &  Co.,  New  York)  ;  and  Toldt's  Atlas  of  Human  Anatomy 
(Rehman  and  Co.,  New  York). 

The  lists   of   references  appended   to   each   chapter   is   merely  typical. 


6  Preface 

There  are  many  books  in  which  the  student  may  find  helpful  material  by 
consulting  the  indices.  The  books  mentioned  above  are  especially  good. 
The  histologies  by  Lewis  and  Stohr  and  by  Bailey  are  well  suited  to  begin- 
ners. Schafer's  Microscopic  Anatomy  (Vol.  II,  Part  I,  of  Quain's  An- 
atomy),  Cunningham's  Anatomy,  Howell's  Physiology,  Starling's  Physi- 
ology, and  Barker's  The  Nervous  System  should  be  in  the  reference  library 
available  to  the  student.  The  Yearly  Psychological  Index  and  the  monthly 
topical  reviews  in  the  Psychological  Bulletin  are  the  most  efficient  guides 
to  recent  psychological  and  psycho-physiological  literature. 

For  the  benefit  of  those  students  to  whom  the  technical  terms  in  the  text 
will  be  new,  the  proper  stress  in  pronunciation  of  some  of  these  terms  is 
indicated  in  the  index. 

I  am  under  obligations  to  a  number  of  persons  who  have  given  me  assist- 
ance in  the  preparation  of  this  book,  especially  to  Dr.  Herbert  M.  Evans, 
Dr.  Caswell  Grave,  and  Dr.  S.  O.  Mast  of  the  Johns  Hopkins  University 
and  Dr.  Percy  W.  Cobb  of  Nela  Research  Laboratory,  who  have  helped 
me  Avith  suggestions  and  criticisms,  and  to  Dr.  Warner  Brown  of  the  Uni- 
versitv  of  California,  Dr.  H.  M.  Johnson  of  Nela  Research  Laboratory, 
and  Dr.  George  R.  Wells  of  Oberlin  College,  who  have  done  the  unpleas- 
ant work  of  reading  the  proof.  K.  D. 

The  Johns  Hopkins  University.  October  15,  1914. 


CONTENTS 


CHAPTER  I 
The  Cell  


CHAPTER    II 
The  Adult  Tissues  of  the  Human  Body  . 


CHAPTER  III 

Muscular  Tissue 27 

The  Function  of  Muscle 33 

Tonus  and  Excitability  ...  36 

The  Contraction  of  Cardiac  Muscle 36 

The  Contraction  of  Smooth  Muscle 37 

The  Chemical  Process  in  Muscle 37 

Fatigue 38 

The  Electrical  Properties  of  Muscle 38 

CHAPTER  IV 

Nervous  Tissue 40 

The  Neuron 42 

The  Structure  and  Investiture  of  Nerve  Fibers  and  Nerves 46 

Gray  Matter  and  White  Matter 53 

The  Cerebro-spinal  and  Sympathetic  Systems 54 

CHAPTER  V 

The  Afferent  and  Efferent  Neurons 55 

The  Afferent  Neurons  of  the  Spinal  Ganglia 55 

Afferent  Nerve  Endings 57 

Free  Nerve  Endings 58 

Tactile  Discs 58 

The  Auditory  and  Gustatory  Endings 58 

Corpuscular  and  Spindle  Organs 60 

Tendon  and  Muscle  Spindles 64 

RufHni's  Endings 66 

Endings  in  Hair  Follicles 66 

Afferent  Neurons  of  the  Olfactory  Membrane 67 

Afferent  Chains  of  the  Optic  Neurons 68 

The  Efferent  Neurons 70 


8  Contents 

PAGE 

CHAPTER  VI 
The   Gross    Relations    of    the    Nerves,    Spinal    Cord,    Brain    and 

Other  Ganglia 73 

Gross  Details  of  the  Brain 77 

The  Columns  and  Tracts  of  the  Spinal  Cord 79 

The  Spinal  and  Cranial  Nerves 82 

CHAPTER  VII 

The  Visceral  or  Splanchnic  Division  of  the  Nervous  System  .    .  86 

Ganglia  of  the  Visceral  System  and  their  Functions 91 

The  Stimulation  of  Afferent  Visceral  Neurons 92 

Referred  Pain 93 

CHAPTER  VIII 

Glands 94 

The  Duct-Glands 95 

The  General  Structure  of  the  Alimentary  Canal 97 

Glands  of  the  Alimentary  Canal ...  98 

Glands  of  the  Skin 103 

The  Ductless  Glands 104 

CHAPTER  IX 

The     Functional     Interrelation     of     Receptors,     Neurons,     and 

Effectors 109 

The  Functional  Unity  of  the  Central  Nervous  System 113 

Reflex  Dominance 114 

Centers  in  the  Brain  and  Cord 115 


AN    OUTLINE   OF   PSYCHOBIOLOGY 


CHAPTER  I. 


THE   CELL. 


The  smallest  unit  of  living  tissue,  both  plant  and  animal,  is  the  cell. 
Every  organism  is  a  cell,  a  group  of  cells,  or  an  aggregate  of  cells  with 
certain  other  structures  produced  directly  by  the  activity  of  cells.  Some 
plants,  and  some  animals,  consist  permanently  of  a  single  cell  each.  Every 
plant  and  every  animal  commences  its  individual  life  as  a  single  cell. 
These  unicellular  organisms  possess,  in  a  limited  way,  the  functioning 
capacities  of  more  complex  organisms.  The  study  of  any  form  or  func- 
tion of  living  tissue  may  therefore  advantageously  begin  in  a  study  of  the 
cell.     [Fig.  1.] 


Fig.  i.  Diagram  of  typical  cell.  (Bailey,  Histology.)  i.  Cell  membrane.  2. 
Granules  of  metaplasm.  3.  Net-knob,  or  karyosome.  4.  Hyaloplasm.  5.  Spongio- 
plasm.  6.  Linin  net-work.  7.  Nucleoplasm.  8.  Attraction-sphere.  9.  Centrosome. 
10.  Plastids.  11.  Chromatin  net-work.  12.  Nuclear  membrane.  13.  Nucleolus.  14. 
Vacuole. 

The  cell  has  been  defined  as  "  a  mass  of  protoplasm  containing  a  nu- 
cleus." 1  It  is  true,  we  find  certain  cells,  the  red  blood  corpuscles  of 
mammals  [Fig.  3]  which,  during  the  period  of  their  special  functional 
activity,  have  no  nucleus.  These  cells,  however,  have  finished  their 
growth,  and  die  without  issue;  during  their  period  of  growth  they  are 
nucleated. 

1  Leydig,  Lehrbuch  der  Histologic  1857,  S.  9. 


12  PSYCHOBIOLOGY 

The  typical  life-history  of  a  cell  has  been  epitomized  in  the  statement 
that  "  both  nucleus  and  cytoplasm  arise  through  the  division  of  the  cor- 
responding elements  of  a  preexisting  cell  ",2  or,  more  succinctly,  "  Omnis 
cellula  e  cellula  ".3 

Protoplasm  is  not  a  single  definite  substance,  but  varies  greatly  from 
cell  to  cell.  The  protoplasm  of  a  nerve  cell,  for  example,  is  different 
from  that  of  a  liver  cell.  The  nucleus  of  any  cell,  moreover,  differs  physi- 
cally and  chemically  from  the  remaining  protoplasm  of  the  cell.  The 
chemical  structure  of  protoplasm  is  in  any  case  exceedingly  complex,  the 
chief  constituents  in  point  of  quantity  being  carbon,  oxygen,  nitrogen, 
hydrogen,  sulfur,  fosforus,  chlorin,  sodium,  potassium,  calcium,  mag- 
nesium, and  iron.  Certain  organisms  include  still  other  elements  in  their 
protoplasm. 


Fig.  2.  Ciliated  cells;  bacilli  of  typhoid  fever.  (Sedgwick  and  Wilson,  General 
Biology.)     An  example  of  an  unicellular  plant. 

Some  cells  are  approximately  spherical  in  shape ;  among  these  are  cer- 
tain eggs,  and  certain  unicellular  plants.  In  most  cases,  however,  the  form 
is  modified  by  growth  in  special  directions,  or  by  pressure  of  surrounding 
cells  or  other  structures.  In  size,  cells  are  usually  microscopic,  the  diam- 
eter of  many  of  the  human  cells  being  as  low  as  four  one-thousandths 
(.004)  of  a  millimeter  (usually  written  4 /x ; read  four  micro  millimeters, 
cr  four  mikrons).4     Human  red  blood  corpuscles  are  quite  uniformly 

2  Schultze,  Arch.  f.  Anat.  u.  Physiol.,  1861,  S.  II. 

3  Virchow,  Arch.  f.  Pathol.  Anat.,  1855,  VIII,  S.  27. 

4  There  is  some  confusion  in  regard  to  the  term  '  micro-millimeter  '.  Certain 
authors  (mainly  physiologists)  use  it  to  designate  the  one-thousandth  part  of  a  milli- 
meter; other  authors  (mainly  physicists)  employ  it  to  signify  the  one-millionth  part 
of  a  millimeter.  Conventionally,  the  prefix  '  micro-',  when  it  is  the  solitary  prefix  to 
the  name  of  a  unit  of  measurement,  means  the  millionth  part  of  that  unit.  One 
micro-volt,  for  example,  is  the  one-millionth  part  of  a  volt.  The  use  of  '  micro-'  before 
another  prefix,  as  in  '  micro-millimeter  '  is  unfortunately  not  standardized. 

The  Greek  letter  corresponding  to  the  initial  of  a  standard  of  measurement  always 
represents  the  one-thousandth  part  of  the  smallest  common  unit  of  that  standard.  Thus, 
fj.   (mu)  indicates  the  thousandth  part  of.  a  millimeter,  and  jifJ-    (mu-mn)  represents  the 


The  Cell 


13 


about  7.5  /a  in  diameter.  Cells  of  voluntary  muscles,  although  but  a  few  //, 
in  diameter,  may  be  several  centimeters  long.  Human  nerve  cells  may  be 
as  much  as  a  meter  in  length,  and  in  larger  animals  there  are  nerve  cells 
of  even  greater  length.  The  largest  single  cells  are  the  yolks  of  birds' 
eggs;  these  cells  do  not  contain  more  protoplasm  than  do  microscopic 
cells,5  but  they  are  swollen  by  the  great  amount  of  foreign  material  present. 


Fig.  3.  Diagram  showing  the  forms  of  certain  of  the  blood  corpuscles  as  enlarged 
1200  diameters.  A,  B,  largest' outline  of  red  blood  corpuscles.  C,  D,  F,  cross  section 
of  red  corpuscles,  perpendicular  to  largest  outline.  Red  corpuscles  as  prepared  on 
microscope  slides  usually  have  the  form  corresponding  to  D,  sometimes  that  corres- 
ponding to  C  or  F.  Histologists  differ  as  to  which  is  the  normal  form  of  the  cor- 
puscles in  the  blood  vessels  of  the  living  animal.  G,  H,  K,  white  corpuscles ;  G,  lym- 
phocyte ;  H,  large  mononuclear  leucocyte  at  rest ;  K,  the  same  in  motion. 

The  protoplasm  within  the  cell,  exclusive  of  the  nucleus,  is  called 
cytoplasm.  Under  examination,  the  cytoplasm  is  sometimes  homogeneous 
in  appearance,  sometimes  it  appears  to  be  finely  granulated,  and  sometimes 
it  appears  to  consist  of  a  reticulated  or  meshed  structure  of  fine  threads, 
the  spong;ioplasm,  the  meshes  filled  with  a  semi-fluid,  the  cytolymph  or 
hyaloplasm.  Some  histologists  hold  that  the  typical  structure  of  the 
cytoplasm  is  alveolar,  that  is,  made  up  of  globular  droplets  separated  from 

millionth  part  of  a  millimeter.  Correspondingly,  a  (sigma)  indicates  the  thousandth 
part  of  a  second,  and  y  {gamma)  if  used  would  represent  the  thousandth  part  of  a 
milligram. 

5  The  protoplasm  and  nucleus  contained  in  the  yolk  of  a  hen's  egg  is  about  1%  of 
the  '  germinal  disc '  which  is  visible  on  one  side  of  the  yolk.  A  human  egg  is 
about  170//  in  diameter. 


14  PSYCHOBIOLOGY 

one  another  by  walls  of  a  different  substance ;  this  is  the  arrangement  of 
the  particles  of  an  emulsion. 

The  nucleus  is  a  body  which  is  in  some  cells  approximately  spherical, 
but  which  in  other  cells  has  a  variety  of  shapes.  It  is  made  visible,  as  are 
the  other  details  of  cell  structure,  by  '  staining  '  the  tissue  prior  to  exami- 
nation under  the  microscope.  The  various  dyes  used  darken  different  por- 
tions of  the  cell  to  different  degrees.  The  nucleus,  however,  is  usually 
visible  without  staining,  because  its  refractive  effect  on  light  is  not  the 
same  as  that  of  the  cytoplasm. 

In  addition  to  the  nucleus  and  the  cytoplasm  proper,  a  cell  usually  con- 
tains certain  other  bodies.  The  most  important  of  these  are  the  centro- 
somes,  which  have  a  function  in  cell  reproduction,  and  plastids  which 
are  concerned  in  the  production  of  various  organic  compounds.  Among, 
the  latter  are  the  amy  I  o  plastids,  or  starch-producing  bodies,  and  the  chloro- 
plastids,  or  chlorophyl-producing  bodies,  of  certain  plant  cells.  In  addi- 
tion, almost  all  cells  contain  foreign  particles,  metaplasm ;  these  may  be 
particles  of  food  not  yet  assimilated,  or  pigment,  or  oil,  or  water,  or  waste 
products  of  cellular  activity,  etc. 

Some  cells,  principally  in  plants,  are  each  surrounded  by  a  cell  wall, 
which  is  produced  by  the  cell.  In  animal  cells,  the  outer  layer  of  cyto- 
plasm constitutes  a  cell  membrane ,  which  is  not  structurally  distinguish- 
able from  the  cytoplasm  adjacent  to  it,  but  which  nevertheless  has  certain 
functional  peculiarities. 

The  nucleus  is  the  controlling  factor  in  the  metabolic  activity  of  the  cell 
(the  breaking-down  of  chemical  combinations,  katabolism,  and  the  build- 
ing-up of  other  combinations,  anabotism) ,  and  upon  it  therefore  depends- 
the  growth  and  the  reproduction  of  cells,  as  well  as  their  other  vital  func- 
tions. It  is  nevertheless  true  that  certain  cells,  when  deprived  of  their 
nuclei,  may  live  for  some  time,  and  perform  such  functions  as  are  gener- 
ally ascribed  to  katabolic  activity ;  may  respond  to  stimulation  by  contrac- 
tion, or  by  transmitting  the  stimulation  to  other  cells,  etc.  Cells  in  which 
anabolic  activity  is  especially  important  (gland  cells),  have  significantly 
large  nuclei,  and  the  nuclear  surface  is  sometimes  rendered  relatively  large 
through  the  formation  of  branches  or  other  irregularities  of  shape. 

The  nucleus,  when  a  section  of  tissue  is  stained  with  certain  dyes,  is- 
darker  than  the  cytoplasm,  and  when  examined  under  a  high-power  micro- 
scope is  seen  to  have  the  dye  absorbed  principally  by  a  certain  portion, 
which  is  for  this  reason  called  chromatin.  In  addition  to  the  chromatin, 
there  is  in  the  nucleus  a  small  rounded  body,  the  nucleolus,  which  also 
stains  deeply,  and  a  reticulum  or  network  of  fine  fibers,  the  linin,  the 
meshes  of  which  are  filled  with  nuclear  juice.     In  some  cells,  the  chro- 


The  Cell 


15 


matin  forms  an  independent,  coarser,  network,  ramifying  through  the  linin 
network ;  in  other  cells  the  chromatin  is  in  the  form  of  granules  distributed 


Fig.  4.  Yeast  cells  budding.  (Sedgwick  and  Wilson,  General  Biology.)  The 
drawings  of  the  successive  stages,  beginning  at  the  left  of  the  top  row,  show  how  the 
bud  forms  from  the  cytoplasm  before  the  nucleus  divides. 


FlG.  5.  Diagram  of  fission  (Amitotic  division)  in  the  unicellular  animal  Parame- 
cium. (Sedgwick  and  Wilson,  General  Biology?)  Paramecium  belongs  to  the  class 
■infusorium  which  have  nuclei  differentiated  into  two  distinct  parts,  viz.,  a  relatively 
large  oval  macronucleus  and  a  much  smaller  micronucleus  lying  beside  it.  In  the 
figure  the  division  of  the  macronucleus  (mac)  and  of  the  micronucleus  (mic)  is 
shown  nearly  completed,  and  division  of  the  cytoplasm  in  progress.  The  Paramecium 
has  a  definite  mouth,  shown  at  m. 


16 


PSYCHOBIOLOGY 


along  the  linin  fibers.  The  distribution  of  the  chromatin  is  in  all  cases 
essentially  modified  during  the  process  of  mitotic  cell  division,  as  described 
below.     Of  the  function  of  the  nucleolus,  practically  nothing  is  known. 

There  are  three  characteristic  ways  in  which  new  cells  are  produced  by 
preexisting  ones.  1.  Budding.  [Fig.  4.]  The  new  cell  grows  directly 
out  of  the  old  one  as  a  twig  grows  out  of  a  limb.  In  this  case  the  parent 
cell  apparently  retains  its  identity,  and  we  can  speak  of  the  new  cell  as 


n.  ,- 


FlG.  6.  Diagram  of  several  successive  stages  in  karyokinesis  (mitotic  division). 
(Schafer,  Microscopic  Anatomy.)  I.  shows  the  'resting'  cell,  i.  e.,  the  cell  before 
the  commencing  of  mitosis.  VIII.  shows  mitosis  virtually  complete,  the  two  new 
cells  being  in  the  '  resting '  condition. 


the  daughter  cell.  The  daughter  cell  is  partially  formed  from  the  cyto- 
plasm of  the  parent  cell,  and  then  a  portion  of  the  parent  nucleus  is 
separated  from  the  remainder  and  passes  into  the  daughter  cell.  2.  Fis= 
sion,  or  direct  division  [Fig.  5].  The  cytoplasm  and  the  nucleus  divide 
by  progressive  constriction,  beginning  in  the  nucleus  so  that  two  daughter 
cells  are  formed,  half  of  the  old  nucleus  going  to  each.     3.  Mitosis,  indi- 


The  Cell  17 

rect  division  or  karyokinesis  [Fig.  6].  This  is  the  more  usual  form  of 
cell  division  in  multicellular  organisms,  and  is  rather  complicated. 
In  contrast  with  mitotic  division  the  first  two  forms  described  above  are 
called  amitotic.  Budding  is  never  found  in  cells  of  the  higher  order  of 
organisms,  and  fission  occurs  in  these  organisms  only  where  the  cells  are 
pathological,  or  are  approaching  the  end  of  their  lines  of  descent  from 
natural  causes,  as  is  the  case  with  the  cells  of  the  membrane  which  lines 
the  bladder,  where  the  cells  are  constantly  being  lost  from  the  surface 
and  replaced  by  others  formed  below. 

In  mitosis,  the  nucleus  takes  up  a  position  near  the  center  of  the  cell, 
and  the  chromatin  forms  a  relatively  thick  thread,  usually  continuous  in 
the  early  stages  of  mitosis.  This  chromatin  thread,  the  skein  or  spireme, 
divides  longitudinally  into  two  nearly  equal  threads,  and  each  of  these 
halves  next  breaks  up  into  a  number  of  short  pieces  which  are  called 
chromosomes.  Meanwhile  the  centrosome,  if  not  double  at  the  begin- 
ning, has  divided,  and  the  two  centrosomes  have  moved  apart  to  positions 
on  opposite  sides  of  the  nucleus.  The  linin  network  at  the  same  time  has 
been  replaced  by  the  mitotic  spindle  of  fine  lines  spreading  out  in  cone 
shape  from  the  centrosomes  and  meeting  midway  between.  (This  is  a 
typical  order  of  events:  in  many  cases  the  sequence  is  different.  For  ex- 
ample: the  lateral  division  of  the  spireme  may  occur  before  the  longitu- 
dinal division:  or  there  may  be  no  spireme  formed.) 

Eventually,  half  of  the  total  number  of  chromosomes  are  drawn  to  each 
centrosome,  where  they  unite  to  form  a  new  skein,  from  which  the  chro- 
matin is  then  distributed  into  its  usual  form  in  the  new  nucleus. 

During  the  formation  of  the  new  nuclei  the  parent  cell  has  begun  to 
constrict  about  the  equator  denned  by  the  polar  axis  through  the  two  cen- 
trosomes. With  the  final  completion  of  this  constriction,  the  original 
parent  cell  is  replaced  by  two  daughter  cells.  The  whole  process  of  mitotic 
division  may  require  from  a  few  minutes  to  several  hours. 

The  presence  of  a  centrosome  can  not  be  demonstrated  in  all  cells.  In 
some  cells,  on  the  other  hand,  there  are  many  centrosomes.  It  is  possible 
that  in  amitotic  division  the  centrosome  plays  an  important  part;  and 
some  investigators  believe  that  it  has  an  important  role  apart  from  its 
function  in  reproduction.  Certain  cells,  such  as  spermatozoa,  some  uni- 
cellular plants  and  animals  [Fig.  2],  and  the  cells  lining  the  respiratory 
passages,  have  fine  cilia,  projecting  externally,  which  are  capable  of  a 
whip-like  motion.  In  some  cells,  it  is  quite  clear  that  these  are  connected 
with  centrosomes,  and  the  analogy  between  the  cilia  and  the  lines  of  the 
mitotic  spindle  has  suggested  that  the  formation  of  such  fibers  is  in  every 
case  the  work  of  centrosomes. 


18 


PSYCHOBIOLOGY 


Fig.  7.  Diagram  showing  an  amoeba  in  three  stages  of  locomotion.  (Jennings, 
Contributions  to  the  Study  of  the  Behavior  of  Lower  Organisms.}  In  a  the  amoeba, 
with  its  pseudopodia  fully  extended  so  that  its  body  is  reduced  to  little  more  than  a 
pseudopodial  conjuncture,  is  floating  in  the  water,  but  one  pseudopod  has  come  in 
contact  with  the  surface  of  a  solid.  In  b  the  protoplasm  has  begun  to  flow  out  of  the 
other  pseudopodia  into  the  one  attached  to  the  solid.  In  e  the  amoeba  is  reduced  to  a 
more  compact  mass,  creeping  along  the  surface.  In  thus  creeping  the  protoplasm  flows 
from  the  larger  portion  into  the  smaller,  and  the  upper  surface  moves  forward,  so 
that  the  motion  is  somewhat  like  that  of  rolling  a  bag  partly  filled  with  a  semi-fluid, 
by  pulling  on  the  front  edge. 


Fig.  8.  Pigment  cell  from  skin  of  frog,  showing  four  stages,  from  A,  complete 
extension,  to  D,  complete  retraction  of  the  pigment.  (Verworn,  Allgemeine  Physi- 
ol ogie.)  It  is  a  question  whether  the  cell-branches  are  retracted  and  extended  (and 
are  therefore  to  be  considered  as  pseudopodia)  or  the  branches  are  fixed  in  form,  and 
only  the  pigment  moves. 

Certain  unicellular  organisms  are  able  to  throw  out  pseudopodia,  or 

leg-like  projections  of  the  protoplasm,  and  retract  them,  thus  assuming 


The  Cell  19 

various  irregular  shapes  [Fig.  8].  By  means  of  these  pseudopodia  cer- 
tain cells  are  enabled  to  move  about;  according  to  one  theory,  using 
them  very  much  like  legs ;  according  to  another  theory,  by  a  process  which 
may  be  described  briefly  if  not  quite  accurately),  as  "  putting  out  a  pseu- 
dopodium  and  then  crawling  into  it."  Cells  which  creep  about  in  this  way 
are  called  wandering  cells,  of  which  there  are  several  sorts  in  the  human 
body,  among  them  the  white  blood  corpuscles,  or  leucocytes  [Fig.  3],  and 
the  osteoblasts  [Fig.  15].  From  the  amoeba  [Fig.  7],  a  typical  unicellular 
animal  found  in  pond-water,  this  method  of  locomotion  is  called  amoeboid 
movement. 

By  means  of  pseudopodic  projections  of  its  cytoplasm  a  wandering  cell 
may  surround  foreign  particles,  which  if  nutritive,  may  be  assimilated 
within  the  cell,  or  if  not,  may  be  carried  to  another  place  and  there  de- 
posited. 

The  activity  of  every  living  cell,  and  therefore  of  every  living  organ- 
ism, is  regarded  as  based  chemically  on  the  processes  of  assimilation  and 
dissimilation.  Assimilation  is  the  conversion  of  food  materials  into  new 
protoplasm ;  dissimilation  is  the  breaking-down  of  protoplasm  into  waste 
products,  and  is  usually  accompanied  by,  or  consists  in  part  in,  oxidization. 
Certain  cells  are  able  to  carry  on,  in  addition  to  assimilation  proper,  the 
synthesis  of  non-living  substances  to  be  stored  up  in  the  cell,  such  as  fat, 
sugar,  and  starch,  or  to  be  cast  out,  as  glandular  secretion.  Such  syn- 
thesis, and  assimilation  likewise,  is  accompanied  by  the  transformation  of 
kinetic  energy  received  from  without  (as  from  the  sun's  rays  in  the  case 
of  plants  producing  starch),  or  from  dissimilation  within  the  cell,  into 
potential  energy.  Dissimilation  liberates  energy,  which  may  be  utilized  in 
anabolic  processes,  or  may  be  available  as  heat  to  maintain  the  temperature 
of  the  organism,  or  may  be  expended  in  work,  as  in  the  contraction  of  the 
muscle  or  the  conduction  of  a  stimulation  in  a  nerve  cell. 

REFERENCES  ON  THE  CELL. 

Wilson,  The  Cell  in  Development  and  Inheritance.     Chapters  I,  II. 

Schafer,    Microscopic    Anatomy    (Vol.    II,    Pt.    I,    of    Quain's    Anatomy,    Eleventh 

Edition),  §  Structure  of  the  Tissues,  Sub-§  The  Animal  Cell. 
Bailey,  Text-Book  of  Histology,  3d  Edition.     Pt.  II.     Chapter  I. 
Lewis  and  Stohr,  A   Text-Book  of  Histology,  Pt.  I.     Chapter  I. 
Starling,  Human  Physiology,  Pt.  I.,  Chapter  II. 

Sidgwick  and  Wilson,  Introduction  to  General  Biology.     Chapters  I-III. 
Hertwig,  Manual  of  Zoology,  (Translated  by  Kingsley),  General  Anatomy,  I. 


CHAPTER  II. 

THE  ADULT  TISSUES  OF  THE  HUMAN  BODY. 

From  the  fertilized  human  egg  cell  there  develops  a  body  composed  of 
cells  differing  widely  from  one  another  in  details  of  structure  and  func- 
tion ;  together  with  certain  non-cellular  structures  produced  and  nourished 
by  cell  activity. 

The  tissues  of  the  body  necessarily  vary  in  structure  with  the  stages  in 
the  development  of  the  individual.  We  are  concerned  directly  with  the 
human  tissues  as  they  exist  in  the  adult,  but  will  find  it  profitable  to  refer 
briefly  to  their  development  within  the  uterus.  Several  classifications  of 
tissue  are  in  vogue,  the  one  here  adopted  being  taken  from  Stohr. 


Fig.  9.  Segmentation  of  the  ovum  (egg),  and  formation  of  the  germ-layer  in  the 
rabbit.  (Lewis  &  Stohr,  Histology.)  A,  two-cell  stage;  B,  four-cell  stage;  C,  mor- 
ula. D-H,  cross-sections  of  later  stages.  Ect.,  ectoderm;  Ent.,  endoderm ;  Ales.,  meso- 
derm. In  G,  the  medullary  or  neural  groove  is  plainly  visible  at  the  top,  and  in  H, 
the  edges  of  the  groove  are  about  to  unite  to  form  the  medullary  tube. 


The  Adult  Tissues  of  the  Human  Body 


21 


The  egg  cell  is  transformed  by  repeated  mitosis  into  the  morula,  a 
spheroidal  mass  of  cells  uniform  in  appearance  [Fig.  9,  C].  From  this 
compact  mass  is  next  developed  the  blastula,  a  hollow  vesicle,  the  cells 
being  distributed  to  form  at  first  a  wall  of  a  single  layer,  and  then  by  con- 
tinued multiplication  forming  three  layers;  the  ectoderm  or  outer  layer, 
the  endoderm  or  inner  layer,  and  the  mesoderm  or  middle  layer. 


Fig.  io.     Stratified  epithelium  from  esophagus  of  cat.     Highly  magnified.     (Bailey, 
Histology.) 

1 


;-tf.v 


M 


•* 


M 
Fig.    II.      Stratified   ciliated    epithelium    from    human    trachea.      Highly    mangified. 
(Bailey,  Histology.)     The  ciliated  cells  are  columnar.     One  'goblet  cell'   (cell  secret- 
ing mucus)   is  shown. 

The  ectoderm  and  the  endoderm  are  epithelia,  composed  of  cells  of 
compact  form  more  or  less  closely  arranged.  The  mesoderm  is  in  part 
made  up  of  cells  like  those  of  the  two  other  layers,  and  in  part  of  branch- 


22 


PSYCHOBIOLOGY 


ing  cells;  mesenchymal  cells;  whose  branches  anastomose,  that  is,  be- 
come joined  together  with  protoplasmic  continuity  from  cell  to  cell,  form- 
ing what  is  called  a  syncytium. 

From  these  three  layers  of  the  blastoderm  develop  the  following  tissues, 
comprising  the  body  of  the  adult  individual. 

1.  Epithelium.  This  is  the  tissue  that  covers  surfaces,  internal  and  ex- 
ternal. The  cells  are  closely  packed,  and  cemented  together  by  substances 
secreted  by  themselves.     An  epithelium  may  be  simple,  composed  of  one 


Fig.  12.  Areolar  connective  tissue;  sub-cutaneous,  from  rabbit.  Highly  magnified. 
(Schafer,  Microscopic  Anatomy.)  The  wavy  bundles  are  white  fibers;  the  straight 
black  lines  forming  an  open  net-work  are  elastic  fibers.  Several  types  of  connective- 
tissue  cells  are  shown  at  e,  /,  g,  and  v. 

layer  of  cells;  or  stratified,  composed  of.  several  layers  [Fig.  10].  The 
cells  from  surface  view  are  usually  polygonal,  and  often  six-sided.  If  the 
depth  of  the  cells  is  approximately  equal  to  their  width,  the  epithelium 
and  its  component  cells  are  both  described  as  cuboidal;  if  the  depth  is 
greater  than  the  width,  they  are  called  columnar  [Fig.  11]  ;  if  the  depth 
is  less  than  the  width,  giving  the  cells  a  flattened  or  scale-like  form,  the 
term  squamous  is  applied. 

Epithelial  cells  may  become  hardened   ( '  cornified  ' ) ,  as  on  the  surface 
of  the  epidermis,  on  the  nails,  and  in  hair.     Other  epithelial  cells  may 


The  Adult  Tissues  of  the  Human  Body 


23 


have  cilia  projecting  from  the  free  surface,  as  in  the  lining  of  the  bron- 
chial tubes,  the  lining  of  the  efferent  ducts  of  the  testes,  and  certain 
cells  of  the  inner  ear.  <  Certain  other  cells  are  active  secreting  organs,  such 
as  the  '  goblet  cells '  which  secrete  mucous. 

The  various  epithelial  tissues  of  the  adult  body  develop  from  all  three 
embryonic  layers. 

2.  Connective  tissue.  This  is  derived  from  the  mesenchymal  cells  of 
the  mesoderm,  and  is  typically  composed  of  cells  with  intercellular  spaces 
largely  filled  with  substances  secreted  by  the  cells,  notably  fibers  of  two 
sorts,  white,  and  elastic.  In  some  connective  tissues  the  bundles  of  fibers 
are  closely  packed  and  are  generally  parallel;  in  others  (areolar6  and 
reticular  connective  tissues),  the  fibers  are  more  loosely  arranged,  and 
run  in  various  directions  [Fig.  12]. 


Fig.  13.  Mucous  connective  tissues  from  umbilical  cord  (navel  string)  of  eight-inch 
foetal  pig.  Magnified  600  diameters.  (Bailey,  Histology.)  Fibers  have  begun  to 
form  in  the  '  ground  substance '  between  the  cells. 

Some  connective  tissue  has  no  well-developed  fibers,  the  intercellular 
spaces  being  filled  with  gelatinous  substance.  This  is  mucous  tissue 
[Fig.  13],  and  in  it  the  cells  show  plainly  the  typical  mesenchymal  form. 


6  Areolar  means  literally  having  spaces  between  the  parts:  hence  loosely  arranged. 
This  word  must  be  distinguished  from  alveolar,  which  means  literally  having  cavities 
cr  cells  (as  honey-comb,  for  example).  Reticular  means  having  the  form  of  a  net,  or 
network 


24 


PSYCHOBIOLOGY 


When  the  cells  of  connective  tissue  become  charged  with  fat  globules,  it 
is  known  as  fat  or  adipose  tissue  [Fig.  14]. 


Fig.  14.     Adipose  sub-cutaneous  tissue  from  dog.     Magnified  200  diameters.     (Ran- 
vier,  Histologie.)     a,  fat  droplets  ;  p,  protoplasm ;  m,  cell-membrane ;  n,  nucleus. 

Tendons,   which  connect  muscles  to  bones,   are  dense  strips  of  con- 
nective tissue  with  parallel  fibers   [Fig.   15].     The  tendon  as  a  whole  is 


FlG.  15.  Longitudinal  section  of  tendon  of  frog.  Magnified  250  diameters. 
(Bailey,  Histology.)  Rows  of  nuclei  of  tendon-cells  are  shown  flattened  between  the 
fibers. 


enclosed  in  a  sheath  of  looser  connective  tissue,  and  the  smaller  bundles 


The  Adult  Tissues  of  the  Human  Body  25 

of  longitudinal  fibers  within  the  tendon  are  also  enclosed  in  sheaths  of  con- 
nective tissue  continuous  with  that  of  the  larger  sheath. 

Wherever  the  connective  tissue  fibers  are  found,  the  cells  which  nourish 


Fig.   16.     Elastic  cartilage   (gristle)    from  human  ear.     Magnified  about  290  diam- 
eters.     (Szymonowicz,  Histologic.)      Showing  cartilage-cells,  and  elastic  fibers. 

Osteoblast  becoming  a  bone  cell.     Bone  cell.    Osteoblast. 

mmMm./  -,  Uncalcified 


'*/ 


- — •  matrix. 


•  Calcified 
matrix. 


Fig.  17.    Part  of  a  cross-section  of  a  bone  from  a  four-month  human  embryo.     Mag- 
rifled  675  diameters.      (Lewis  and  Stohr,  Histology.) 

them  are  found  also.     In  the  dense  tissues  the  cytoplasm  and  nucleus  may 
be  flattened  between  fibers  or  in  thin  plates  wrapped  around  them. 

3.  Bone  and  cartilage  are  produced  by  mesenchymal  cells.     Bone  is 
deposited  by  a  specialized  cell,  the  osteoblast,  which  as  it  encloses  itself 


26  PSYCHOBIOLOGY 

within  the  bone  it  manufactures,  is  called  a  bone  cell  [Fig.  17].  Car- 
tilage is  formed  as  the  thick  cell  walls  secreted  by  the  cartilage  cells 
[Fig.  16]. 

4.  Nerve  tissue  comprises  the  '  nerves  ',  spinal  cord,  brain,  and  other 
ganglia,  and  is  derived  wholly  from  the  ectoderm. 

5.  Muscle.  Smooth  or  non-voluntary  muscle  develops  from  the 
mesenchymal  cells  of  the  mesoderm.  Striped  or  voluntary  muscle  de- 
velops from  the  non-mesenchymal  cells  of  the  mesoderm. 

6.  Vascular  tissue.  This  includes  the  blood  and  lymph,  the  lymph 
glands  and  the  red  marrow  of  the  bones,  and  develops  from  the  endoderm. 

7.  Glands.  The  active  tissues  in  these  are  composed  of  modified  epi- 
thelial cells.     Glands  are  developed  from  all  three  blastodermic  layers. 

Nervous,  muscular,  and  glandular  tissues  will  be  treated  more  fully  in 
the  following  chapters. 

REFERENCES  ON  TISSUES  GENERALLY. 

Schafer,  Microscopic  Anatomy,  §  Structure  of  the  Tissues,  Sub-§§  The  Elementary 
Tissues,  The  Epithelial  Tissues,  Connective  Tissue. 

Bailey,  Histology,  Part  III,  Chapters  I-III. 

Lewis  and  Stohr,  Histology,  Pt.  I,  §  II,  Sub-§§  Histogenesis,  Epithelium,  Mesen- 
chymal Tissue. 

Hertwig,  Manual  of  Zoology,  §  General  Anatomy,  II. 


CHAPTER  Hi. 

MUSCULAR  TISSUE. 


BUCCINATOR 


o.mo-hyoid 
Sterno-hyoid 


ThYREO-HYOID 


Crico-thyreoid, 


Tensor  veli-palatini 

MUSCLE 

Eustachian  tube 

•Levator  veli  palatini 

.Pterygomandibular 

ligament 

Superior  constrictor 

)t  ylo -ph  a  r  yng  eus 
.Stylo-glossus 
Glosso-pnarvngeal 
nerve 

Stylo-hyoid  ligament 
Hypoglossal  nerve 
Middle  constrictor 
Digastric 
Superior  laryngeal 
nerve 

Inferior  constrictor 


External  laryngeal 
nerve 


CEsophagus 
Interior  laryngeal 
aerve 


Fig.   i  8 


]■     Lateral  view  of  the  wall  of  the  pharynx,  showing 
in  vocalization,     fr^-n-n^^-u <      .  ,     & 


involved  in  vocalization.     (Cunningham,  Anatomy.)     The 


small  capitals 


some  of  the  muscles 
names  of  the  muscles  are  in 


28 


PSYCHOBIOLOGY 


The  muscles  of  the  human  body  are  usually  classified  under  three  heads. 

1.  Striated  (striped)   or  'voluntary'. 

2.  Non=striated  (non-striped),  smooth,  or  '  non-voluntary '. 

3.  Cardiac   (heart-muscle),  striped,  but  non-voluntary. 

Striated  muscle  tissue  forms  the  skeletal  muscles,  that  is,  the  muscles  of 
the  body  wall  and  of  the  limbs ;  and  also  the  muscles  of  the  eye  and  ear, 


F- 


Fig.  19.  Parts  of  two  striped  muscle  fibers  of  dog.  Magnified  270  diameters. 
(Ranvier,  Histologie.)  The  fibers  have  been  broken  without  breaking  the  sarco- 
lemma.    n,  nucleus;  s,  sarcolemma;  p,  fluid  between  sarcolemma  and  muscle-cell. 

the  diaphragm,  the  tongue,  pharynx,  larynx,  upper  part  of  the  esophagus, 
and  in  part  of  the  rectum  and  genital  organs. 

Striated  muscles  develop  from  certain  of  the  non-mesenchymal  cells 
of  the  mesoderm.    These  cells,  which  are  called  myoblasts,  divide  by  re- 


Muscular  Tissue  29 

peated  mitosis  and  become  elongated,  and  then  shift  to  the  positions  cor- 
responding to  those  which  are  to  be  occupied  by  the  muscles  in  the  mature 
body.  The  mesenchymal  cells  adjacent  to  the  muscles  form  tendons  and 
muscle  sheaths  and  the  fascia  (broad  sheets  of  connective  tissue  lying  be- 
tween the  muscles  and  the  skin) . 

The  myoblasts  consist  of  granular  cytoplasm,  called  sarcoplasm,  hav- 
ing fibrils  near  the  periphery  and  centrally  located  nuclei,  and  are  enclosed 
in  a  delicate  connective  tissue  membrane,  the  sarcolemma.  The  myo- 
blasts may  have  a  diameter  of  from  IO/a  to  100 fi. 

The  fibrils  within  the  myoblast  divide  longitudinally,  and  group  them- 
selves, forming  muscle  columns  .3/x  to  .5//,  in  diameter,  between  which 
lies  the  sarcoplasm. 

The  nucleus  of  each  myoblast  divides  amitotically  into  many  nuclei, 
which  scatter  along  the  cell,  which  has  now  become  a  muscle  fiber 
[Fig.  19],  a  single  cell  which  may  be  from  50  to  120  mm.  in  length,  and 
from  10  jm    to  50/x  in  diameter. 

In  pale  or  white  muscle  the  columns  of  fibrils  nearly  fill  the  fiber,  with 
thin  layers  of  sarcoplasm  between  them,  and  the  nuclei  are  generally  flat- 


Capillaries 


Bundles   of   fibrils 
(Cohnheim's  areas) 


£fH^fifiiii£>> 


Connective  tissue 

Fig.  20.  Cross-section  of  four  fibers  of  human  vocalis  muscle,  showing  the  group- 
ir.g  of  the  myofibrils  to  form  Connheim's  areas.  Magnified  590  diameters.  (Lewis 
and  Stohr,  Histology.') 

tened  in  between  the  sarcolemma  and  the  fiber.  A  few  nuclei  are  found 
lying  inside,  among  the  bundles  of  fibrils  towards  the  end  of  the  fiber, 
and  particularly  towards  the  distal  end,  at  which  point  elongation  in 
growth  takes  place.  In  dark,  or  red  muscle,  there  are  fewer  fibrils  and 
more  sarcoplasm,  and  the  nuclei  are  located  more  centrally,  embedded 
among  the  fibrils.     Connective  tissue,  including  cells  with  indefinite  out- 


30 


PSYCHOBIOLOGY 


lines  and  flattened  nuclei  smaller  than  the  nuclei  of  the  muscle  fibers,  not 
only  surrounds  each  fiber,  but  surrounds  small  bundles  of  the  fibers, 
bundles  of  these  bundles,  and  the  whole  muscle.  In  cross  section  this  con- 
nective tissue  forms  a  continuous  network,  enveloping  and  reticulating 
within  the  muscle  [Fig.  20]. 

The  sheath  of  the  whole  muscle  is  the  external  perimysium,  the  con- 
nective tissue  within  the  muscle  is  the  internal  perimysium  [Fig.  21]. 
Lymph  and  blood  vessels  and  nerves  are  found  in  the  internal  perimysium. 
The  nerve  terminals  are  either  on  the  surface  of  the  ordinary  muscle  fibers, 
or  on  fibers  enclosed  in  muscle  spindles. 


External  perimysium. 


Muscle  bundles. 


/internal  perimysium. 


Cross  section  of  artery. 


Muscle  spindle. 


Cross  section  of  nerve.' 


Muscle  of  Man..    X  6o. 


Fig.  21.  Cross-section  of  human  striped  muscle  {omohyoid),  magnified  6o  diam- 
eters.    (Lewis  and  Stohr,  Histology.) 

The  individual  fibrils  are  composed  of  two  kinds  of  substance,  arranged 
in  regular  alternation.  The  isotropic  substance  singly  refracts  light,  and 
does  not  readily  stain  in  the  preparation  of  sections.  The  anisotropic 
substance  is  doubly  refractive,  and  stains  deeply.  The  arrangement  of 
these  two  substances  in  the  fibrils,  of  a  single  cell  is  practically  parallel, 
and  this  gives  the  '  striped  '  appearance  to  the  fibers.     The  functions  of 


Muscular  Tissue 


31 


these  two  substances  is  not  understood,  but  there  are  several  tentative 
theories  as  to  the  way  in  which  they  behave  in  the  process  of  contraction. 

The  muscle  fibers  are  rounded  or  conical,  the  end  towards  the  tendon 
being  more  obtuse  than  the  other.  Connection  with  the  tendon  is  provided 
by  the  perimysium,  which  is  continuous  with  the  tendon  [Fig.  22].  The 
connection  of  a  muscle  with  a  tendon,  or  with  other  tissues,  at  its  rela- 
tively moveable  end,  is  called  its  insertion;  the  attachment  at  the  relatively 
fixed  end  of  the  muscle  is  its  origin.  In  the  cases  of  muscles  inserted  in 
the  skin,  or  the  mucous  membrane,  the  ends  of  the  fibers  may  be  branched 
or  pointed,  the  perimysium  being  prolonged  as  elastic  fibers  which  be- 
come continuous  with  the  connective  tissue  in  the  skin  or  membrane. 


.    Sarcoplasm. 


Muscle  nucleus.  •    / 
Transition  zone.  ; 


Tendon  fiber  bundles. 


Sarcolemm 


Tendon  nucleus. 


Fig.  22.     Junction  of  striped  muscle  and  tendon  in  frog. 
(Bailey,  Histology,  after  Stohr.) 


Magnified  750  diameters. 


Smooth  muscle  develops  from  mesenchymal  cells.  It  is  found  sur- 
rounding the  large  blood  vessels  and  lymphatic  ducts,  the  intestinal  canal 
and  the  ducts  of  the  principal  gland  opening  into  it,  the  large  respiratory 
tubes  and  the  passages  and  ducts  of  the  genito-urinary  system,  and  subcu- 
taneously  in  connection  with  hairs.  It  may  for  convenience  (although 
not  with  exact  accuracy)  be  called  visceral  muscle,  in  contradistinction  to 
skeletal  muscle. 


32 


PSYCHOBIOLOGY 


The  fibers  of  smooth  muscle  are  elongated  spindle-shaped  cells,  each 
with  a  single  nucleus  centrally  located  [Fig.  24].  The  fibers  vary  in 
length  from  20 /a  to  500jaand  are  typically  about  5  ^  in  diameter.  Within 
these  cells  '  coarse  '  longitudinal  fibers,  composed  entirely  of  anisotropic 


■-  ■.--.     u--,-:-r,iV.'v?3^ 


FlG.  23.  Smooth  muscle  in  longitudinal  section  of  small  intestine.  Magnified  350 
diameters.  (Bailey,  Histology.)  The  inner  circular  layer  (at  top),  traversely  cut, 
and  the  outer  longitudinal  layer  are  shown  with  the  intermuscular  septum  of  con- 
nective tissue  between.     The  cross-section  of  a  small  artery  is  visible  in  the  septum. 

[i.  e.,  doubly  refracting),  substance  develop  near  the  surface.  These 
border  -fibrils  are  believed  to  be  continuous  from  cell  to  cell.  Surrounded 
by  the  border  fibrils  are  fine  inner  -fibrils,  which  separate  to  pass  around 
the  centrally  located  nucleus.     Smooth  muscle  fibers  are  covered  by  con- 


FiG.  24.  Isolated  smooth  muscle  cells  from  human  small  intestine.  Magnified  400 
diameters.  (Bailey,  Histology.)  The  centrally  located  nuclei  are  visible  as  oval, 
darker  areas. 


nective  tissue  in  much  the  same  way  as  are  the  striped  muscles,  except  that 
two  or  more  cells  in  longitudinal  contiguity  may  be  enveloped  in  the  same 
sheath. 

The  muscle  of  the  vertebrate  heart  belongs  to  the  type  designated  as 
cardiac.  The  fibrils  of  cardiac  muscle  are  composed  of  alternated  sections 
of  isotropic  and  anisotropic  substances,  but  the  cells  have  usually  a  single 
centrally  located  nucleus,  like  the  smooth  muscle  cell.  According  to  some 
observers,  the  cardiac  muscle  does  not  consist  of  individual  cells,  but  is  a 
syncytium,  that  is,  a  tissue  in  which  there  is  protoplasmic  continuity 


Muscular  Tissue 


33 


irom  cell  to  cell,  so  that  the  limits  of  the  individual  cells  can  not  be  exactly 
assigned.  This  anastomosis  (union  of  cells)  is  not  confined  to  the  lon- 
gitudinal direction,  but  occurs  between  lateral  surfaces  also  of  contiguously 
lying  cells,  so  that  heart  muscle,  according  to  this  view  is  "  a  network  of 
broad  protoplasmic  bands,  in  and  near  the  centers  of  which  nuclei  are 
situated  at  irregular  intervals"    (Stohr)    [Fig.  25].     According  to  other 


fJ'i 


m 


S^tt  tig, 


:- 


f  J     mm 


I'IG.  25.  Muscle  fibers  of  heart,  showing  syncytial  structure.  Highly  magnified. 
(Schafer,  Microscopic  Anatomy,  after  Przewoski.)  a,  septum;  b,  fibrils  bridging 
septum ;  c,  nucleus ;  d,  short  segment  without  nucleus. 

observers,  there  is  merely  longitudinal  continuity  of  fibrils,  not  of  proto- 
plasm, the  apparent  anastomosis  being  produced  in  the  process  of  prepar- 
ing the  section  of  tissue  for  observation.  The  groups  of  mesenchymal  cells 
from  which  smooth  and  cardiac  muscles  develop,  do,  however,  show  well- 
marked  anastomosis  between  their  branches. 

Mesenchymal  cells,  in  becoming  cardiac  muscle  cells,  lose  their  original 
power  of  reproduction.     Possibly  striated  muscle  can  reproduce. 

THE  FUNCTION  OF  MUSCLE. 

The  most  important  characteristic  of  the  muscle  cell  is  its  highly-devel- 
oped power  of  contraction.  Many  other  cells  are  able  to  contract,  or  to 
change  their  shape  in  various  ways;  but  in  the  muscle  cell,  contractile 


34  PSYCHOBIOLOGY 

function  is  developed  to  the  highest  degree.  The  beating  of  the  heart,  the 
regulation  of  the  diameters  of  blood  vessels,  the  inflation  of  the  lungs,  the 
movement  of  food  along  the  alimentary  canal,  the  erection  of  the  body 
hairs  in  '  gooseflesh  ',  the  movements  of  the  limbs  and  other  portions  of  the 
body,  are  produced  by  the  properly  timed  contractions  of  myriads  of 
muscle  cells. 

In  contracting,  the  muscle  cells  become  thicker  and  shorter,  and  undergo 
internal  changes  which  are  not  well  understood.  During  this  process, 
toxic  substances,  having  decided  effects  on  both  muscular  and  nervous 
tissue,  are  formed.  In  its  resting  phase  the  cell  carries  on  metabolic  pro- 
cesses which  maintain  its  own  life,  produces  material  essential  to  the 
process  of  contraction,  and  possibly  produces  substances  of  value  to  other 
tissues. 

Under  ordinary  circumstances  a  striated  muscle  contracts  only  when  it  is 
stimulated  by  some  external  agency.  Normally,  the  stimulation  is  an 
impulse  conveyed  to  the  muscle  fiber  from  a  ganglion  cell  through  its 
axon,  whose  terminal  branches  are  in  contact  with  the  sarcoplasm  beneath 
the  sarcolemma.  Contraction  can  be  produced,  however,  by  mechanical, 
electrical,  chemical,  or  thermal  stimuli  applied  directly  to  the  muscle.  A 
muscle  from  the  leg  of  a  frog,  for  example,  contracts  if  pinched  by  a  pair 
of  tweezers,  or  if  an  electric  current  be  made  or  broken  through  it,  or  if 
ammonia  or  certain  salt  solutions  be  applied  to  it.  If  the  muscle  be 
gradually  warmed,  it  commences  to  contract  at  about  34°  C.  (93°  F. ), 
the  contraction  increasing  up  to  about  45°  C.  (113°  F.),  at  which  point 
the  muscle  dies,  although  the  contraction  lasts  some  time  longer  as  rigor 
caloris. 

When  a  single  stimulation  is  applied  to  a  striated  muscle  there  is  a  brief 
latent  period  after  the  stimulus,  and  then  the  muscle  responds  with  a 
single  short  sharp  contraction,  relaxing  immediately.  The  duration  of  the 
latent  period,  contraction  and  relaxing  vary  with  the  intensity  of  the  stim- 
ulus, and  with  the  temperature,  tonus,  (tone;  vide  infra,  VIII),  and 
fatigue  of  the  muscle,  and  with  the  work  done.  The  frog's  gastroc- 
nemius muscle,  at  ordinary  room  temperature  may,  when  excited  by  an 
induction  coil  current,  give  such  results  as:  latent  period,  10  cr;  contraction, 
40  a  relaxing,  50  cr;  the  whole  process  therefore  taking  place  within^  sec. 
(1  second  is  1000 cr).  In  the  living  animal,  when  the  muscle  has  tone. 
the  times  may  be  much  shorter. 

The  amount  of  shortening  which  a  striated  muscle  displays  is  influ- 
enced by  the  same  conditions  which  control  the  time  relations  of  the  pro- 
cess. Under  similar  conditions  of  temperature,  fatigue,  etc.,  a  weak  stimu- 
lus may  cause  a  slight  contraction,  a  stronger  stimulus  a  more  pronounced 
contraction.     There  is,  of  course,  a  maximum  for  earh  muscle. 


Muscular  Tissue  35 

The  same  muscle  under  otherwise  similar  conditions,  will  contract  less, 
if  the  '  load  '  or  work  done  be  greater.  In  the  excised  frog's  muscle,  the 
upper  end  may  be  rigidly  supported,  and  a  weight  attached  to  the  lower 
end.  The  extent  of  contraction  will  then  decrease  with  increasing  '  load  '. 
If  both  ends  be  rigidly  fastened,  we  may  produce  '  contraction  '  without 
shortening  •  the  muscle  being  under  longitudinal  tension  during  the  moment 
of  activity. 

When  stimulated  by  ordinary  means,  the  whole  muscle  fiber  does  not 
contract  simultaneously.  Contraction  begins  at  the  point  at  which  the 
stimulus  (whether  nerve  current  or  artificial  stimulus)  is  applied,  and 
spreads  in  both  directions.  The  rate  of  propagation  of  the  contraction  is 
3  to  4  meters  per  second  in  the  frog's  muscle,  and  may  be  as  high  as  6 
meters  per  second  in  muscles  of  warm-blooded  animals.  The  contraction 
is  strictly  limited  to  the  fibers  stimulated ;  and  does  not  directly  affect 
adjacent  fibers.  The  contraction  of  a  fiber  may,  however,  cause  contrac- 
tion of  other  fibers  in  contact  with  branches  of  the  same  nerve  axon,  the 
stimulation  of  one  branch  by  the  contraction  of  the  muscle  fiber  in  contact 
with  it  being  transmitted  to  the  other  branches  and  from  them  to  the  fibers. 

When  successive  stimulations  are  applied  to  a  muscle  at  such  rate  ( 1 5 
to  40  per  second,  according  to  conditions),  that  before  the  contraction 
produced  by  one  has  ceased,  another  has  occurred,  the  result  is  a  single 
contraction,  maintained  as  long  as  the  stimulations  continue,  and  much 
greater  in  extent  than  the  contraction  which  would  be  produced  by  a  single 
stimulus  of  the  series.  Such  a  state  of  contraction  is  called  tetanus.  If  the 
stimuli  are  applied  at  a  rate  too  slow  to  produce  tetanus,  but  so  that  the 
successive  stimuli  arrive  before  the  effect  of  the  preceding  one  has  com- 
pletely disappeared  (i.  e.,  before  the  end  of  the  period  of  relaxing),  the 
result  is  a  series  of  contractions  much  greater  in  extent  than  the  contrac- 
tion resulting  from  a  single  stimulus  of  the  same  sort.  This  condition  is 
called  summation  of  contractions.  Again,  stimuli  so  weak  that  singly  they 
produce  no  contraction,  may,  when  given  at  a  sufficiently  rapid  rate,  pro- 
duce contraction.  This  condition  is  known  as  the  summation  of  stimuli. 
It  is  supposed  that  the  '  voluntary  '  contraction  of  muscle  is  tetanic,  due 
to  a  rapid  succession  of  nervous  discharges.  There  is  some  evidence  that 
these  discharges  to  the  muscle  occur  at  a  rate  of  40  per  second. 

The  only  continuous  stimuli  are  those  of  the  normal  nerve  activity,  heat, 
and  chemical  action.  Electric  currents  are  effective  only  at  the  moment 
of  making  or  breaking  the  current.  Mechanical  pressure  is  effective  onlv 
at  the  moment  wdien  the  pressure  is  increased  or  decreased. 


36  PSYCHOBIOLOGY 

TONUS  AND  EXCITABILITY. 

The  constant  stimulation  which  is  supplied  by  the  nerves  in  the  normal 
body  keeps  the  muscles  in  a  state  of  normal  contraction  (tonus),  the  de- 
gree varying  continuously  with  the  changes  in  the  flux  of  nervous  current. 
This  continuous  stimulation  also  heightens  the  sensitivity  of  the  muscle  to 
the  more  pronounced  and  definitely  directed  currents  which  bring  about 
the  coordinated  contractions  which  produce  movements ;  or  in  other  words, 
increases  the  irritability  or  excitability  of  the  muscle.  If  the  efferent 
nerves  supplying  any  of  the  muscles  be  severed,  the  muscles  relax  com- 
pletely and  become  motionless,  except  in  so  far  as  artificial  stimulation 
(e.  g.}  electrical)  may  be  employed  upon  them. 

In  the  case  of  cardiac  and  smooth  muscle,  and  probably  also  in  the  case 
of  striped  muscle,  certain  nerve  currents  have  an  action  which  is  inhibi= 
tory,  i.  e.,  the  reverse  of  excitatory.  Certain  nerve  fibers,  as  for  example 
the  fibers  of  the  vagus  which  supply  the  heart  have  inhibitory  action  only. 
Whether  in  all  cases  inhibitory  currents  are  carried  by  special  inhibitory 
fibers,  or  whether  both  excitatory  and  inhibitory  currents  may  be  carried  by 
certain  fibers,  is  unknown. 

The  factors  upon  which  the  irritability  of  muscle  depends,  are  (in  addi- 
tion to  stimulation  and  tonus  of  nervous  origin),  temperature,  condition  of 
rest  or  fatigue,  and  activity  of  various  adventitious  chemical  substances. 
The  greater  the  fatigue,  the  less  the  excitability.  Salts  of  sodium  increase 
the  excitability,  and  calcium  salts  lower  it.  By  immersing  a  thin  striated 
muscle  (e.  g.,  the  sartorius  of  the  frog),  in  a  solution  of  NaCl  0.5%, 
Na2HP04  0.2%,  and  Na2C03  0.04%c  ("  Biedermann's  fluid"),7  it  is 
thrown  into  a  state  of  excitability  which  shows  a  certain  likeness  to  that 
of  smooth  and  cardiac  muscle.  The  muscle,  in  this  solution,  contracts  re- 
peatedly, and  may  '  beat '  rhythmically  like  heart  muscle,  but  at  a  more 
rapid  rate. 

THE  CONTRACTION  OF   CARDIAC  MUSCLE. 

Cardiac  muscle,  in  its  normal  condition,  like  striated  muscle  in  Bieder- 
mann's solution,  contracts  periodically  without  a  periodic  stimulus.  The 
heart  of  a  living  animal '  receives  excitatory  currents  from  nerves  of  the 
'  sympathetic  '  system,  and  inhibitory  currents  from  the  vagus  nerve ;  but 
these  do  not  cause  the  periodic  activity  of  the  heart  muscle.  In  reptiles 
and  many  other  animals,  the  heart  continues  its  contractions  after  being 
completely  isolated  from  the  nervous  system,  and  even  after  being  re- 
moved from  the  body.     If  the  excitability  of  the  excised  heart  be  increased 

7  Formula  given  by   Starling,  Phys'.ology,  page    235. 


Muscular  Tissue  37 

by  immersing  it  in  Biedermann's  solution  or  one  of  certain  other  saline 
solutions,  the  beating  may  be  prolonged  for  hours. 

The  stimulus  producing  the  contraction  is,  in  the  case  of  the  excised 
heart,  internal ;  such  can  be  considered  to  be  the  case  in  the  undetached 
living  heart.  The  nerve  currents  serve  but  to  increase  (or  decrease)  the 
excitability  of  the  cardiac  muscle,  and  hence  to  accelerate  (or  retard)  the 
spontaneous  activity.  Chemicals  which  modify  the  excitability  of  the  ex- 
cised heart  also  modify  the  excitability  of  the  heart  in  situ.  Cardiac 
muscle,  in  addition  to  its  power  of  periodic  contraction  without  periodic 
stimulation  from  outside,  and  even  without  any  external  stimulation,  re- 
sponds to  a  single  external  stimulus  by  a  single  twitch,  as  does  striated 
muscle.  The  contraction  of  the  heart  muscle  so  brought  about  differs 
from  the  corresponding  contraction  of  striated  muscle  in  three  particu- 
lars :  ( 1 )  The  excitation  may  pass  from  fiber  to  fiber  directly,  because  of 
the  protoplasmic  connections  between  fibers;  (2)  The  degree  of  contrac- 
tion is  not  dependent  upon  the  intensity  of  the  stimulus.  If  a  fiber  con- 
tracts at  all  it  contracts  with  the  full  force  of  which  it  is  capable.  A 
stimulus  which  will  not  produce  full  contractions  will  produce  no  contrac- 
tions at  all;  (3)  Summation  of  contraction  and  tetanus  do  not  occur  in 
the  case  of  the  cardiac  muscle.  While  the  muscle  is  contracting,  a  stimu- 
lus has  no  effect  on  it.  After  the  contraction,  it  becomes  again  irritable. 
The  interval  during  which  the  muscle  is  unexcitable  is  called  the  refractory 
period. 

THE   CONTRACTION  OF   SMOOTH   MUSCLE. 

Smooth  muscle  resembles  cardiac  muscle  in  having  active  power  of  its 
own.  Cut  off  from  all  nervous  connection,  it  still  may  contract  and  relax 
alternately  if  subjected  to  a  continuous  external  stimulus,  tension,  for  ex- 
ample. In  general  smooth  muscle  responds  to  all  the  artificial  stimuli  to 
which  striated  muscle  responds,  and  it  also  responds  to  various  drugs,  such 
as  digitalis,  ergot,  salts  of  barium,  etc.,  which  produce  different  effects  on 
smooth  muscle  of  different  organs. 

Smooth  muscle  is  supplied,  like  cardiac  muscle,  with  a  double  set  of 
nerves,  one  heightening  the  excitability  and  the  other  depressing  it. 

THE  CHEMICAL  PROCESS  IN  MUSCLE. 

The  chief  products  of  muscular  activity  are  carbon  dioxid,  water,  and 
sarcolactic  acid.  Sarcolactic  acid  is  isomeric  with  the  lactic  acid  of  sour 
milk,  but  the  former  rotates  the  plane  of  polarization  of  polarized  light  to 
the  right,  whereas  the  latter  does  not  rotate  the  plane  at  all.  It  is  prob- 
able that  the  primary  chemical  activity  which  conditions  muscular  con- 
traction is  the  breaking-down  of  some  complex  substances,  which  may  be 


38  PSYCHOBIOLOGY 

highly  unstable,  although  one  physiologist  supposes  it  to  be  grape  sugar 
(C6H1206).  The  lactic  acid  (C3H603)  is  then  oxidized,  the  oxygen  being 
supplied  by  the  red  blood  corpuscles.  Oxidization  is  at  any  rate  an  im- 
portant part  of  the  chemical  process  in  muscle,  and  through  it  is  liberated 
the  heat,  or  at  least  a  part  of  the  heat,  which  is  a  noticeable  consequence 
of  muscular  activity. 

Normally,  the  greater  part  of  the  lactic  acid  produced  is  oxidized  in  the 
muscle.  If  sufficient  oxygen  is  not  present  the  acid  is  thrown  in  abnormal 
quantity  into  the  circulation,  and  excreted  by  the  kidneys.  In  normal 
urine,  from  3  to  4  milligrams  of  acid  per  hour  are  excreted.  In  one  case, 
the  urine  passed  thirty  minutes  after  running  a  third  of  a  mile  contained 
454  mgs.  of  lactic  acid. 

In  the  intervals  of  '  rest '  the  muscle  cell  builds  up  the  complex  sub- 
stance which  is  broken  down  in  the  period  of  'activity'.  According  to 
some  physiologists  the  food  material  and  oxygen  are  together  built  up  into 
an  unstable  compound,  which  is  broken  down  in  activity,  forming  C02, 
without  additional  oxygen.  According  to  this  theory,  the  account  given 
above  is  erroneous. 

FATIGUE. 

If  a  muscle  be  repeatedly  stimulated,  its  contractions  eventually  become 
less  in  extent,  and  the  latent  periods,  as  well  as  the  periods  of  contraction 
and  relaxation — especially  the  latter — become  prolonged.  In  the  human 
being  this  condition  is  accompanied  by  the  experience  of  fatigue. 

Whatever  may  be  the  exact  nature  of  the  chemical  processes  in  muscular 
activity,  it  is  probable  that  two  factors  contribute  to  fatigue :  ( 1 )  the 
partial  exhaustion  of  the  stored  material  which  the  cell  has  elaborated  as 
material  for  its  contractile  activity;  (2)  the  accumulation  of  the  products 
of  decomposition.  These  substances  (lactic  acid,  carbon  dioxid,  etc.)  have 
an  inhibitory  or  deadening  effect  on  the  muscle  cell,  and  possibly  some  of 
them  affect  the  sensory  nerve  endings  in  the  muscles,  producing  the  experi- 
ence of  fatigue.  In  the  body  the  circulation  of  blood  both  brings  fresh 
food  material,  to  be  built  up  into  new  muscle  material,  and  also  removes 
the  waste  products.  If  the  muscle  activity  is  relatively  great,  however, 
the  waste  product  cannot  be  removed  as  fast  as  formed,  and  either  the 
food  material  is  not  brought  fast  enough,  or  else  the  muscle  cell  cannot 
fast  enough  build  up  muscle  material  out  of  it. 

THE   ELECTRICAL    PROPERTIES   OF   MUSCLE. 

Under  certain  conditions  an  electric  current  may  be  drawn  off  from  a 
muscle  by  applying  suitable  electrodes  to  it.  If  one  electrode  (A)  is  ap- 
plied to  an  uninjured  portion  of  a  resting  muscle,  and  the  other  (B)  to  a 


Muscular  Tissue  39 

cut  or  otherwise  injured  portion,  a  delicate  galvanometer  in  circuit  with  A 
and  B  will  show  a  very  small  current  flowing  from  A  to  B.  This  is  spoken 
of  as  the  '  current  of  demarcation  '  or  '  resting  current '. 

If  electrodes  are  applied  to  an  uninjured  muscle,  at  some  distance  from 
each  other,  and  the  muscle  is  caused  to  contract,  a  current  flows  during 
contraction  from  the  electrode  at  which  the  degree  of  contraction  is  lowest 
to  the  electrode  at  which  it  is  highest.  If,  therefore,  the  muscle  be  stimu- 
lated at  a  point  M,  near  one  end,  the  electrode  A  being  applied  at  the 
middle  of  the  muscle  and  the  electrode  B  at  the  other  end,  when  the  exci- 
tation wave  reaches  A,  the  current  will  flow  from  B  to  A,  and  when  the 
wave  reaches  B,  the  current  will  flow  in  the  reverse  direction.  This  cur- 
rent is  known  as  the  '  current  of  action  '  or  '  action  current '. 

The  currents  for  muscle  may  not  be  of  any  special  significance.  They 
are  in  any  case  probably  artifactual,  that  is,  there  is  probably  no  current 
unless  electrodes  are  applied  and  an  external  circuit  established  through 
them.  In  the  case  of  the  '  resting  current '  there  may  not  even  be  a  differ- 
ence of  potential  between  the  cut  and  uninjured  portions  before  the  elec- 
trodes are  applied. 

REFERENCES  ON  MUSCLES. 

Bailey,  Histology,  Pt.  Ill,  Chapter  V. 

Lewis  &  Stohr,  Histology,  Pt.  I,  §  II,  Sub-§  Muscular  Tissue. 

Shafer,  Microscopic  Anatomy,  §  Structure  of  the  Tissues,  Sub-§  Muscular  Tissue. 

Sanderson,    J.    B.,   The    Mechanical,    Thermal,    and    Electrical    Properties    of   Striped 

Muscles.     Schafer's  Text-Book  of  Physiology. 
Howell,  A   Text-Book  of  Physiology,  Chapters  I  &  II. 
Starling,  Physiology,  Chapter  V. 


CHAPTER  IV. 


NERVOUS  TISSUE. 


Nervous  tissue  develops  from  the  ectoderm.  At  an  early  stage  in  the 
development  of  the  embryo,  after  it  has  elongated,  a  dorsal  longitudinal 
groove,  the  medullary  groove  (or  neural  groove)  [Fig.  26],  forms, 
and  the  edges  of  this  groove  soon  come  together,  forming  the  medullary 
tube  (neural  tube)  [Fig.  27].  The  walls  of  the  anterior  part  of  this 
tube  become  later  very  thick,  forming  the  brain  [Fig.  28]  ;  with  relatively 
small  cavities,  the  ventricles,  representing  the  original  cavity  of  this  part 
of  the  tube.     The  walls  of  the  posterior  part  of  the  tube  thicken  to  a  less 


-SpMWp 

EN    / 


FlG.  26.  Transverse  section  of  ferret  embryo,  showing  medullary  (neural)  groove. 
(Cunningham,  Anatomy.)  EC,  ectoderm.  EN,  endoderm.  GC,  germinal  cell.  N, 
r.otochord.  NG,  neural  groove.  PM,  paraxial  mesoderm.  SpM,  splanchnic  meso- 
derm.   SoM,  somatic  mesoderm.    Sb,  spongioblast. 


SB      NC      CC 


IMC  N  PA 

Fig.  27.  Transverse  section  of  ferret  embryo  of  greater  age  than  shown  in  Fig.  26, 
showing  medullary  canal.  (Cunningham,  Anatomy.)  NC,  neural  crest.  CC,  cen- 
tral canal.    SG,  spinal  ganglia. 


Nervous  Tissue 


41 


degree,  but  more  uniformly,  forming  the  spinal  cord,  with  a  small  central 
canal. 

As  the  medullary  tube  forms,  some  cells  pass  outwardly  from  the  pos- 
terior portion  forming  the  spinal  ganglia  (to  be  described  later).  At  an 
early  period,  not  definitely  determined,  other  cells  migrate  from  the  grow- 
ing spinal  cord   (or  possibly  from  the  spinal  ganglia),  forming  the  sym- 


CEPHALIC 
FLEXURE 


Fig.  28.  Profile  view  of  the  brain  of  a  human  embryo  of  six  weeks.  (Modified 
from  Cunningham,  after  His.)  A,  myelencephalon.  B,  metencephalon.  C,  isthmus. 
D,  mesencephalon.     E,  diencephalon.     F ,  telencephalon. 

pathetic  ganglia,  and  other  visceral  ganglia,  more  remote  from  the 
cord  than  are  the  spinal  ganglia.  From  the  anterior  part  of  the  medullary 
tube  cells  also  migrate  to  the  structures  which  become  the  cochlea  of  the 
ear,  and  the  retina  of  the  eye,  and  possibly  to  the  sensory  surfaces  of  the 
organs  of  smell  and  taste.8 

From  the  brain,  spinal  cord,  spinal  ganglia,  and  visceral  ganglia  the 
nerve  cells  send  processes  (nerve  fibers)  to  the  various  tissues  of  the  body. 


8  It  has  been  a  question  whether  the  four  autonomic  or  visceral  ganglia  in  the  head 
are  formed  by  the  migration  of  cells  from  the  anterior  part  of  the  medullary  tube, 
or  from  the  sympathetic  ganglia  of  the  trunk.  Recent  investigations  point  to  forma- 
tion from  both  sources. 


42 


PSYCHOBIOLOGY 


Fig.  29.  Cerebro-spinal  axis,  reduced  to  J/g  diameter.  (After  Bougery.)  A  longi 
tudinal  section  through  the  median  plane  of  the  spinal  column,  and  in  part  of  the 
skull ;  leaving  in  relief  the  brain,  cord  and  spinal  nerve-roots.  Lobes  of  the  cerebrum  : 
Fa,  parietal ;  F,  frontal ;  O,  occipital ;  T,  temporal  [compare  Fig.  65]  ;  C,  cerebellum ; 
mo,  medulla  oblongata;  Ci-Cviii,  cervical  nerves;  Dl-Dxn,  thoracic  nerves  (formerly 
known  as  dorsal  nerves)  ;  Ll-Lx,  lumbar  nerves;  Sl-Sv,  sacral  nerves;  Co,  coccygeal 
T.erve ;  ms,  ms,  upper  end  lower  extremities  of  the  spinal  cord. 

THE  XEURON. 

The  essential  elements  of  the  nervous  system  are  the  nerve  cells  with 
their  prolongations  1  fibers).  The  nerve  cell,  exclusive  of  the  prolonga- 
tions, is  called  the  celNbody.  The  nucleus  lies  within  the  cell-bod}'.  The 
cell-body  and  the  prolongations  together  are  known  as  the  neuron.  There 
are  two  general  kinds  of  fibers,  designated  axon  and  dendrite  (or  den= 
dron).  whose  differences  will  be  described  later. 


Nervous  Tissue 


43 


The  chief  function  of  the  neuron  (aside  from  nourishing  itself)  is 
conduction.  So  far  as  is  now  known,  the  importance  of  the  neuron  for  the 
total  organism  is  the  fact  that  it  can  receive  stimulation  from  (can  be  irri- 
tated by)  another  neuron  or  by  an  extra-neural  agency;  and  can  transmit 
this  stimulation  through  itself  to  another  neuron,  or  to  a  muscle  or  gland. 


■zSSM 


Fig.  30.  Nerve  cell  from  the  cere- 
bral cortex.  Magnified.  (Ramon  y 
Cajal.)  e,  axon,  c,  collaterals  of  axon. 
a,  b,  dendrites  (dendrons).  P,  teleo- 
dendrites,  or  terminal  branches  of  prin- 
cipal dendrite. 


Fig.  31.  Motor  cell  from  ventral  horn 
of  gray  matter  of  rabbit's  spinal  cord. 
Highly  magnified.  (Barker,  Nervous 
System,  after  Nissl.)  The  process 
(fiber)  extending  directly  downwards  is 
the  axon ;  the  others  are  dendrites. 


Beyond  this  activity  (and  self -nourishment)  we  have  no  evidence  of  any 
other  important  function  of  the  nerve  cell.  In  its  development  the  power 
of  conduction,  which  is  possessed  by  less-highly  specialized  cells  (and  even 
by  muscle  cells  which  are  specialized  in  a  different  direction)  has  reached 


44 


FSYCHOBIOLOGY 


the  highest  degree  of  efficiency.  Coincidentally,  the  nerve  cell  has  lost 
the  power  of  contraction,  and  like  muscle  and  other  highly-specialized 
cells,  the  power  of  reproduction. 

A  neuron  as  a  whole  can  conduct  in  but  one  direction.  It  receives  im- 
pulses through  the  dendrites  (of  which  there  may  be  several)  and  sends 
them  out  through  the  axon  (each  cell  having  one  axon). 

Taking  the  cell-body  as  a  center  of  reference,  we  may  say  that  the  den- 
drites conduct  in,  and  the  axon  conducts  out.  The  only  possible  difference 
in  conduction  between  fibers  (i.  e.,  axon  and  dendrites)  and  the  cell-body, 
lies,  however,  in  the  fact  that  the  '  current '  or  '  discharge  '  may  become 
intensified  in  passing  through  the  cell-body. 


Fig.  32.  Synaptic  connections  of  axon-branches  with  cell-bodies  in  the  cerebellum. 
Highly  magnified.  (Ramon  y  Cajal.)  b,  c,  axon  of  cell  B,  with  branches  in  contact 
with  '-cells  of  Purkinje  '  in  row  at  right  of  A. 


As  to  the  nature  of  the  nerve  current,  i.  e.,  what  passes  when  the  neuron 
is  irritated  at  one  end,  and  shortly  thereafter  at  its  other  end  irritates  an- 
other neuron  or  a  gland  or  muscle  cell,  we  can  merely  guess.  Probably  it 
is  a  chemical  process,  analogous  to  the  action  in  a  train  of  powder,  which, 
when  lighted  at  one  end,  carries  the  combustion  process  to  the  other  end. 

It  is  customary  to  classify  the  neurons  as  (1)  sensory,  (2)  motor,  and 
( 3 )  associative,  according  as  they  .  ( 1 )  receive  a  stimulation  from  a  non- 
neural  source,  (2)  transmit  a  stimulation  to  a  muscle  or  gland,  or  (3) 
transmit  between  other  neurons.    A  better  terminology  for  the  three  classes 


Nervous  Tissue 


45 


of  neurons  is  (1)  'centripetal'  or  afferent,  (2)  'centrifugal'  or  effer= 
ent,  and  (3)  'intermediate'  or  central.  The  peculiarity  of- the  various 
parts  of  the  brain  and  spinal  cord,  which  are  divided  more  or  less  com- 
pletely into  laterally  symmetrical  halves,  makes  it  necessary  to  further 
divide  the  intermediate  neurons  of  these  structures  into  two  classes:  (a) 
those  which  run  across  from  one  half  of  brain  or  cord  to  the  other  half, 
(b)  those  lying  entirely  in  one  half,  or  in  one  half  and  peripheral  struc- 
tures. The  (a)  class  are  called  commissural  neurons,  and  the  name  asso- 
ciation neurons  refers  strictly  to  the  (b)  class. 


Fig.  33.  Diagram  of  the  simplest  possible  reflex  arc  from  epidermis  through  cere- 
bral cortex  to  striped  muscle.  (Cunningham,  after  Ramon  y  Cajal.)  The  arc,  as 
drawn,  involves  four  neurons.  According  to  some,  there  is  another  synapse  between 
E  and  the  cortex,  in  the  midbrain.  There  may  be  intermediate  neurons  in  the  cortex, 
-forming  links  between  the  afferent  neuron  E  and  the  efferent  neuron  A. 


It  is  generally  believed  that  the  neurons  are  distinct  individuals  or  struc- 
tural units,  and  that  where  several  neurons  form  a  functional  series  or 
chain,  the  axon  of  one  cell  is  merely  in  contact  with  the  next  cell,  or  with 
its  dendrite.  These  points  of  contact  between  neurons ;  the  points,  that  is. 
at  which  the  stimulus  is  passed  on  from  one  to  the  other,  are  called 
synapses  (singular  either  synapse  or  synapsis),  or  synaptic  points 
[Figs.  32-34]. 


46 


PSYCHOBIOLOGY 


Both  axons  and  dendrites  may  have  many  branches,  like  the  rootlets  of 
a  tree.  Thus,  in  certain  parts  of  the  nervous  system,  the  terminations  of 
one  axon  may  be  in  contact  with  the  dendrites  or  the  cell-bodies  of  a  num- 
ber of  other  neurons,  and  conversely,  the  branches  of  a  dendrite  may  be  in 
contact  with  axon  branches  of  several  cells.  Moreover,  as  many  cells 
have  many  dendrites  each,  the  multiplicity  of  possible  connection  from  cell 
to  cell  is  very  important. 

Although  an  axon  cannot  conduct  a  stimulus  to  a  dendrite  of  the  sam^ 


mw% 


FlG.  34.  Diagrams  of  reflex  mechanisms  in  the  spinal  cord.  (Barker,  Nervous 
System,  after  Kolliker.)  Left:  two-neuron  arc,  s,  sg,  sa,  sc,  afferent  neuron  with 
ganglion  cell  (sg),  and  ascending  and  descending  branches  and  collaterals  within  the 
cord,  m,  n,  efferent  neuron,  with  cell  bodies  in  cord.  Right :  three-neuron  arc.  Only- 
one  collateral   (sc),  of  the  afferent  neuron  is  shown,     c,  c,  associative  neuron. 

cell,  a  stimulation  may  be  received  by  one  branch  of  an  axon  and  trans- 
mitted to  other  branches  of  the  same  axon.  Thus,  if  a  single  muscle  fiber 
is  caused  to  contract  it  may  irritate  the  axon  branch  applied  to  it,  and  cause 
the  contraction  of  another  fiber  supplied  by  another  branch  of  the  same 
axon. 


THE  STRUCTURE  AND  INVESTITURE  OF  NERVE  FIBERS  AND  NERVES. 

The  fibers  (axons  or  dendrites)  consist  of  longitudinal  fibrils  em- 
bedded in  a  protoplasm  which  is  called  neuroplasm.  These  fibrils  seem 
to  run  through  the  cell-bodies,  and  thus  the  fibrils  in  axon  and  dendrites 


Nervous  Tissue 


47 


Fig.  35.  Portions  of  two  medullated  axons  from  rabbit.  Magnified  425  diameters. 
(Schiifer,  Microscopic  Anatomy.')  R,  R,  nodes  of  Ranvier,  dividing  the  medullary 
sheaths  into  segments,  a,  neurilemma,  c,  nucleus  and  cytoplasm  of  neurilemma.  The- 
myelin  is  stained  black  in  this  preparation. 


48 


PSYCHOBIOLOGY 


would  be  continuous.  This  is  not  certainly  established.  All  nerve  fibers 
which  pass  beyond  the  immediate  vicinity  of  their  cell-body  seem  to  be 
provided  with  one  or  more  coverings.  Three  types  of  coverings  have  been 
distinguished.  A.  A  coat  of  delicate  cells  whose  origin  is  from  the  neural 
crest.  This  is  the  neurilemma,  or  sheath  of  Schwann.  B.  A  connec- 
tive tissue  sheath:  the  sheath  of  Henle.  C.  A  coating  of  myelin,  a  fat- 
like substance  held  in  suspension  in  a  network  of  another  substance  called 
neurokeratin.    Some  histologists  have  described  a  fourth  type  of  sheath. 

It  is  probable  that  all  fibers  have  a  neurilemma,  except  those  in  the 
'  gray  matter  '  of  the  nerve  centers.  In  all  cases,  however,  the  neuri- 
lemma is  absent  for  a  short  distance  after  leaving  the  cell-body,  and  again 
at  the  farthest  end.  Most  observers  claim  that  fibers  in  the  white  matter 
of  the  brain  and  cord  have  no  neurilemma:  Ramon  y  Cajal,  however,  finds 


■t  cells. 


Artery. 


Epineurium. 


Perineurium. 


Bundles  of  nerve  fibers. 


Fig.  36.  Cross-section  of  a  portion  of  a  human  nerve.  Magnified  about  20  diam- 
eters. (Lewis  and  Stohr,  Histology.)  Seven  funiculi  are  shown;  these  are  composed 
of  bundles  of  medullated  nerve  fibers  with  irregular  septa  of  endoneurium  and  are 
wrapped  in  lamellar  fibrous  perineurium.  The  areolar  epineureum  which  binds  the 
funiculi  together  contains  many  fat  cells. 


the  neurilemma  on  some  of  those  fibers.  All  fibers  in  the  '  white  '  matter 
of  the  brain  and  cord,  and  most  fibers  elsewhere,  have  the  myelin  (and 
neurokeratin)  sheath  for  a  portion  of  their  course,  as  least,  and  are  called 
medullated  fibers.  The  fibers  which  have  not  the  myelin  sheath  are  called 
non=medullated,  these  latter  belonging  chiefly  to  the  visceral  system. 

The  myelin  sheath,  when  it  is  present,  always  lies  next  to  the  axon, 
under  the  neurilemma  (if  the  latter  is  present).  The  connective  tissue 
sheath,  which  is  found  only  in  peripheral  fibers,  is  outside  the  neurilemma. 


Nervous  Tissue  49 

Medullated  nerve  fibers  have  the  myelin  deposit  interrupted  annularly 
at  intervals  of  from  80^  to  a  millimeter  along  it;  the  interruptions  are 
known  as  the  nodes  of  Ranvier  [Fig.  35]  ;  all  branches  of  medullated 
fibers  appear  at  these  nodes.  As  there  is  but  one  nucleus  for  every  inter- 
nodal  segment  of  the  myelin  sheath,  such  segments  are  thought  by  some 
histologists  to  be  each  developed  from  a  single  cell,  the  neurilemma  cor- 
responding to  the  cell-membrane,  and  the  neurokeratin  to  the  spongio- 
plasm. 

The  nerve-fibers  are,  except  at  their  terminations,  associated  in  bundles 
-called  nerves.  These  are  spoken  of  as  medullated  and  non-medullated, 
according  as  they  are  made  up  of  medullated  or  non-medullated  fibers. 
The  larger  medullated  nerves  are  made  up  of  a  number  of  bundles  (called 
funiculi  or  fasciculi),  each  surrounded  by  a  sheath  of  dense  connective 
tissue  (perineurium)  which  is  continuous  with  the  sheath  of  Henle  sur- 
rounding each  fiber,  the  whole  nerve  being  surrounded  by  a  layer  of  loose 
■connective  tissue,  the  epineurium   [Fig.  36]. 

The  spinal  cord  lies  in  the  cervical,  thoracic  (or  'dorsal')  and  lum= 
Dar  portions  of  the  spinal  canal  of  the  vertebral  or  spinal  column  [Fig. 
38].  The  spinal  column  is  composed  of  33  vertebrae  [Fig.  39]  articu- 
lated together,  each  vertebra,  except  the  lower,  false  or  fixed  vertebrae, 
Tiaving  back  of  its  '  body '  a  vertical  opening,  somewhat  annular  in  cross- 
section,  the  spinal  foramen.  These  foramina  are  segments  of  the  spinal 
•canal,  the  walls  of  which  are,  therefore,  partly  the  foramen  walls  and 
partly  the  ligaments  uniting  the  vertebrae. 

The  '  pedicles  '  of  the  cervical,  thoracic,  and  lumbar  vertebrae  (exclu- 
sive of  the  upper  onss,  the  '  atlas  '  and  the  '  axis  ')  joining  the  '  body  '  of 
each  to  its  posterior  portion,  are  notched,  more  deeply  so  on  the  under 
side  ( '  inferior  notch  ' ) ,  so  that  as  the  vertebrae  are  articulated,  the  '  in- 
ferior notch  '  of  one  and  the  '  superior  notch*  of  the  one  below  it  form  a 
lateral  opening,  the  intervertebral  foramen,  through  which  run  the 
nerves  connecting  the  spinal  cord  with  the  trunk  and  limbs  (the  spinal 
nerves,  of  which  there  are  31  pairs). 

The  spinal  ganglia  of  all  the  spinal  nerves,  except  four  pairs,  lie  in  the 
intervertebral  foramina  [Fig.  40].  Thus  they  are  protected  by  the  bony 
column.  The  ganglia  of  the  sacral  and  the  coccygeal  nerves  lie  in  the 
spinal  canal  itself,  and  the  ganglia  of  the  first  and  second  cervical  nerves 
lie  on  the  '  arches  '  of  the  first  and  second  vertebrae. 

The  spinal  cord  is  composed  mostly  of  *  white '  matter  surrounding  a 
•core  of  '  gray '  matter,  the  latter  having  in  cross-section  roughly  the  shape 
of  the  letter  H  [Fig.  41].  Outside  the  'white'  matter  are  three  pro- 
tective membranes   [Fig.  40]:    (1)    the  pia  mater,  a  fibrous  connective 


50 


PSYCHOBIOLOGY 


Pons  (Varoli)  ^ 


Radix  n.  ab* 
duccntis 


Oliva 

Pyramis  --^Sl 

Radix  anterior  --"^m 
n.  cervicalis  I.      ^ 

Radices  ante- 

riores  nn.cer- 

vicalium 

Intumescentia 
cervicalis 

Fissura  mediana 
anterior 
Funiculus 
anterior 

Funiculus 
lateralis 

Radices  ante- 
riores  nn.  thora-  ''V 
calium 


Radices  ante- 


Intumescentia 

lumbalis 
Radices 

anteriores 
nn.  sacraliura 

Radix  anterior 
n.  coccygei 


r  Radix  h.  hypoglossi 


Decussatio 
pyram'idum 


Fig.  38.  Vertebral  column,  left  view- 
Reduced  to  about  one-third  diameter. 
(Cunningham,  Anatomy.) 

Fig.  37.  Spinal  cord,  anterior  view.  Reduced  to  about  one-half  diameter.  (Toldt, 
Anatomischer  Alias.)  Showing  the  roots  (radices)  of  the  spinal  nerves,  the  lumbar 
and  cervical  enlargements    (Intumescentiae),  the  pons,  olives,  and  pyramids. 


Nervous  Tissue 


51 


tissue  coat,  closely  applied,  continuous  with  the  inner  surface  of  which 
are  fine  septa,  penetrating  the  '  white  '  matter;  (2)  the  arachnoid,  a  thin 
membrane  loosely  wrapped  around  outside  the  pia  mater,  leaving  an  inter- 


. Superior 
articular  process    Pedicle 


Facet  for 

tubercle  of  rib 

(Fovea  costalis 

transversalis) 

Transverse, 
process 


Demi-facet  for  head  of  rib 
(Fovea  costalis  superior) 
Body 


Inferior      Inferior        Demi-facet  for 
articular      notch  head  of  rib 

process  (Fovea  costalis 

inferior) 

Mrous  process 


Spinous  process 


Facet  for 
tubercle  of 
rib 
(Fovea  cos- 
talis trans- 
versalis) 

Superior  articular 
process 


Pedicle 

Demi-facet  for  head 

of  rib  (Fovea  costalis 

inferior) 

Body 


Fig.  39.     Fifth  thoracic  vertebra,  actual  size.     (Cunningham,  Anatomy.')     A,  right 
view.    B,  from  above. 


val  (the  '  subarachnoid  space  ')  filled  with  '  cerebrospinal  fluid  ' ;  and  (3) 
the  dura  mater,  forming  a  dense  tubular  sheath,  considerably  larger  than 
the  cord,  and  extending  downwards  below  the  limit  of  the  cord  into  the 


52 


PSYCHOBIOLOGY 


sacral  part  of  the  canal.    The  cord  is  attached  to  the  inner  surface  of  the 
sheath  of  dura  mater  by  two  lateral  wing-like  ligamenta  denticulata. 


Duram  ater  spinalis  . 


Posterior  septum 

Posterior  root 


Ligamentum  denticulatum 
nterior  root 


Subarachnoid 
Pia  mater 


Anterior  branch 


Posterior  branch 


Ramus  communicans 


Fig.  40.    Cross-section  through  fourth  cervical  vertebra,  showing  cord  and  its  cover- 
ings.    Enlarged  about  two  diameters.     (Bailey,  Histology,  after  Rauber-Kopsch.) 


^olumna  posterior 
Hintersaule) 


Columna  lateralis 

(Seitensaule) 


Columr*a  anterior 
(Vordersaule) 


Fig.  41.     Cross-section  through  spinal  cord  of  adolescent,  at  level  of  first  cervical 
nerve.     Magnified  two  diameters.     (Toldt,  Anatomischer  Atlas.) 


Nervous  Tissue 


53 


Between  the  dura  mater  and  the  wall  of  the  spinal  canal  is  a  small  space 
filled  by  fatty  tissues  and  blood  vessels. 

On  the  anterior  and  posterior  sides  of  the  cord  there  are  fissures,  into 


Fig.  42.  Ependymal  and  neuroglia  cells  in  embryonic  spinal  cord.  Enlarged. 
(After  v.  Lenhossek.)  A,  central  canal.  B,  B,  ependymal  cells;  modified  neuroglia 
ctlls  composing  the  epithelium  lining  the  canal ;  only  a  few  are  stained.  C,  C,  typical 
neuroglia  cells  in  the  gray  matter  of  the  cord. 

each  of  which  a  fold  of  the  pia  mater  extends,  nearly  dividing  the  cord 
into  two  halves.  A  fold  of  the  arachnoid  (the  septum)  extends  to  the 
dorsal  side  of  the  cord,  across  and  dividing  the  subarachnoid  space. 

GRAY  MATTER  AND  WHITE  MATTER. 

The  '  white  matter  '  of  the  brain,  cord  and  nerves  is  made  up  chiefly  of 
medullated  nerve  fibers,  running  through  the  framework  of  neuroglia. 


54  PSYCHOBIOLOGY 

The  '  gray  matter  '  (found  only  in  brain,  cord  and  ganglia)  is  composed 
of  cell-bodies,  and  non-medullated  fibers,  in  a  neuroglia  framework. 

Neuroglia  is  composed  of  branched  or  nbrillated  cells  (neuroglia  cells ; 
derived  from  the  same  embryonic  layer  as  the  nerve  cells)  whose  branches 
anastomose,  forming  a  reticular  tissue  or  network  [Fig.  42]. 

There  appear  to  be  two  kinds  of  neuroglia  cells,  in  the  one  of  which  the 
processes  branch  repeatedly,  in  the  other  of  which  (the  '  spider  cells')  the 
processes  are  entirely  unbranched. 

THE  '  CEREBROSPINAL  '  AND  '  SYMPATHETIC  '  SYSTEMS. 

It  is  customary  to  class  together  the  neurons  whose  cell  bodies  lie  in  the 
brain,  cord,  spinal  ganglia,  ganglia  of  the  cranial  nerve  roots,  and  '  sense 
organs ',  as  the  cerebro=spinal  system.  The  neurons  whose  bodies  lie 
in  the  other  ganglia  ('sympathetic'  ganglia)  are  classed  as  the  sympa= 
thetic  or  autonomic  system.  This  classification  is  useful  if  it  is  under- 
stood that  there  are  not  two  separate  systems,  but  two  intimately  associated 
parts  of  one  system.  Classification  and  terminology  in  respect  to  these 
divisions  of  the  total  nervous  system  are  not  well  agreed  upon.  We  shall 
refer  to  this  point  later. 

REFERENCES  ON  NERVE  TISSUES. 

Barker,  The  Nervous  System,  Chapters  I-XXV. 

Howell,  Physiology,  Chapters  III  and  V. 

Starling,  Physiology,  Chapter  VI,  §§  I-VI. 

Bailey,  Histology,  Chapter  V. 

Schafer,  Microscopic  Anatomy,  §  Structure  of  the  Tissues,  Sub-§§  The  Tissues  of  the 

Central  Nervous  System. 
Cunningham,  Anatomy,  §  The  Nervous  System,  Sub-§§  The  Cerebrospinal  System,  and 

The  Spinal  Cord. 
Lewis  and  Stohr,  Histology,  Part  I,  §  II,  Nervous  Tissue. 
Schafer,  The  Nerve  Cell.     In  Schafer's  Physiology. 


CHAPTER  V. 

THE  AFFERENT  AND  EFFERENT  NEURONS. 

The  cell-bodies  of  the  afferent  neurons  (so-called  '  sensory  '  neurons) 
are  found  in :  ( 1 )  the  spinal  ganglia,  the  ganglia  of  the  cranial  nerve 
roots,  and  the  sympathetic  and  collateral  ganglia.  Here  are  located  the 
bodies  of  the  neurons  whose  dendrites  extend  to  the  skin,  subcutaneous 
tissues,  muscles,  bones  and  tendons  and  viscera  of  the  body;  (2)  the  retina 
of  the  eye,  the  cochlea  of  the  ear,  and  the  olfactory  membrane  of  the  nose. 

THE  AFFERENT  NEURONS  OF  THE  SPINAL  GANGLIA. 

The  afferent  neuron  cells  in  the  spinal   ganglia  are  modifications  of 


Posterolateral 
croove 


Anterior  nerve-root 
Posterior  nerve-root 


Spinal  ganglion 

Anterior  primary 
division  of  nerve 
Posterior  primary 
■M  division,  of  nerve 


Fig.  43.      Portion   of  cord,   showing   the   roots   and   spinal   ganglia   of  the   seventh 
thoracic  nerve.     Enlarged  about  two  diameters.     (Cunningham,  Anatomy.) 


bipolar  cells,  i.  e.,  cells  having  one  axon  and  one  dendrite  growing  from 
approximately  opposite  sides ;  in  the  process  of  development  the  axon  and 


56 


PSYCHOBIOLOGY 


the  dendrite  move  together,  and  fuse  for  a  short  distance  of  their  length,, 
giving  rise  to  a  T-shaped  process  [Fig.  44]  ;  hence  these  cells  are  some- 
times called  T=cells,  and  (incorrectly)  '  unipolar  '  cells.  (There  are  true 
unipolar  cells  found  elsewhere.) 

The  axon  of  the  cell  is  sent  into  the  spinal  cord  through  the  posterior 
root.  Each  posterior  root,  on  entering  the  cord,  divides  into  two  bundles. 
The  smaller  bundle  passes  to  the  outer  side  of  the  tip  of  the  posterior 
horn  ('  Lissauer's  '  tract)  where  each  fiber  bifurcates,  one  branch  running 
up  the  cord  and  the  other  down.     These  branches  run  only  a  short  dis- 


FiG.  44.  Bipolar  cells  in  a  spinal  ganglion  of  an  embryo.  Highly  magnified. 
(Schafer,  Microscopic  Anatomy,  after  Ramon  y  Cajal.)  A,  B,  T-cells.  E,  E,  cells 
still  retaining  the  typical  bi-polar  form.  C,  D,  F,  G,  cells  in  process  of  transition 
from  the  bi-polar  to  the  T-form. 

tance,  sending  off  lateral  branches  which  penetrate  the  gray  matter  and 
arborize  around  cells  there. 

The  larger  bundle  of  the  posterior  root  fibers  passes  to  the  inner  side  of 
the  horn  and  enters  the  posterior  column,  the  fibers  bifurcating,  and 
one  branch  passes  up  and  the  other  down  as  described  for  the  fibers  of  the 
smaller  bundle.     The  ascending  branches  of  some  of  these  fibers  run  up 


The  Afferent  and  Efferent  Neurons 


57 


to  the  medulla.  The  ascending  branches  of  the  other  fibers  run  up  a  short 
distance  and  then  into  '  Clark's  column '  at  the  bases  of  the  horns,  arbor- 
izing there  around  cell-bodies  whose  axons  run  up  to  the  cerebellum.  Cer- 
tain of  the  posterior  root  fibers  arborize  around  cells  in  the  anterior  horn,. 
i.  e.,  motor  cells. 

All  these  ascending  and  descending  branches  send  lateral  branches  into 
the  gray  matter  of  the  cord,  as  do  the  fibers  of  the  other  bundle. 

The  dendrites  of  some  of  the  T-cells  probably  pass  from  the  nerve  over 
the  white  ramus  communicans  to  the  sympathetic  and  collateral  ganglia ; 
whether  these  fibers  continue  without  interruption  through  the  ganglia  to- 
the  visceral  tissues,  or  whether  they  are  relayed,  receiving  stimulations 
from  cells  in  the  sympathetic  or  collateral  ganglia,  does  not  seem  clearly 
made  out.  The  other  dendrites  pass  out  over  the  spinal  nerves  to  termin- 
ations in  skin,  subcutaneous  tissues,  muscle  and  tendon,  and  are  properly 
called  somatic  afferent  neurons. 


Fig.  45.     Free  nerve  endings  in  the  epithelial  lining  of  the  esophagus  of  a  rabbit. 
Highly  magnified.     (Barker,  Nervous  System,  after  Retzius.) 


AFFERENT  NERVE  ENDINGS. 

The  dendrons  of  afferent  neurons  end  in  four  ways:  1.  'free';  2.  in 
contact  with  specially  adapted  epithelial  cells ;  3.  in  special  structures  or 
'  end  organs  '  of  connective  tissue,  '  encapsulated  endings ' ;  and  4.  the 
dendrite  in  certain  cases  is  modified,  forming  a  characteristic  end  organ 
which  is  a  part  of  the  neuron  itself. 


■58  PSYCHOBIOLOGY 

FREE   NERVE   ENDINGS. 

The  fibers  are  said  to  end  free  when  there  are  no  specific  '  end  organs ' 
in  connection  with  them  [Fig.  45].  Free  endings  are  found  chiefly  in 
epithelial  tissues  (skin,  mucous  membrane,  cornea  of  eye,  etc.),  although 
they  may  be  present  in  other  tissues. 

When  afferent  fibers  terminate  in  the  '  free '  way,  they  usually  branch 
several  times  in  sub-epithelial  tissues,  and  lose  first  (as  we  approach  the 
epithelium)  the  connective  tissue  sheath,  then  the  medullary  sheath,  and 
finally  the  neurilemma.  Then,  nearer  the  epithelium,  the  branches  of  sev- 
eral fibers  form  a  '  skein  '  or  network  of  fibrils  called  primary  plexuses. 
From  these  plexuses  branches  are  given  off  which  form  secondary  plexuses, 
still  nearer  the  epithelium.  Finally,  fibrils  proceed  from  the  secondary 
plexus  and  ramify  among  the  epithelial  cells.  The  actual  nerve  endings, 
or  endings  of  the  ultimate  branches,  are  '  free  varicose  fibrils  '. 

TACTILE  DISCS. 

In  some  cases  (in  deep  layers  of  stratified  epithelium)  the  fibrils  termin- 
ate in  flattened  or  saucer-shaped  plates,  tactile  discs  [Fig.  46],  applied 
to  epithelial  cells  (called  tactile  cells)  as  is  the  cup  to  an  acorn. 


Fig.  46.     Tactile  discs.     Highly  magnified.     (Ramon  y  Cajal.)     These  flat  expan- 
sions of  the  fibers,  containing  net-works  of  fibrils,  lie  between  the  cells  in  epithelial 

tissues. 

The  tactile  discs  in  the  human  being  are  especially  numerous  in  the  skin 
over  the  thighs  and  abdomen. 

THE   AUDITORY  AND   GUSTATORY   ENDINGS. 

The  dendrites  of  the  neurons  located  in  the  spinal  ganglia  of  the  cochlea 
pass  out,  through  the  spiral  lamina,  and  their  branches  are  applied  to  the 


The  Afferent  and  Efferent  Neurons 


59 


hair  cells  [Fig.  47].  These  hair  cells  are  columnar  epithelial  cells,  from 
the  free  extremities  of  which  bundles  of  cilia  (auditory  hairs)  project. 
These  cells  are  specialized  receptors  for  auditory  stimuli,  and  pass  the 
stimulus  on  to  the  dendritic  branches  in  contact  with  them.  The  auditory 
hair  cells  are  arranged  in  a  single  or  double  row  on  the  '  inner '  side  of  the 
organ  of  Corti ;  and  a  triple  or  quadruple  row  on  the  '  outer  '  side.  There 
are  possibly  from  20,000  to  25,000  of  these  hair  cells  in  an  ear  in  the 


Membr.  tector. 


Lam.    spir. 


Vas  spir. 


Membr.    basil. 


Fig.  47.  Cross-section  of  the  organ  of  Corti,  showing  nerve  endings  in  the  cochlea. 
Magnified  probably  500  diameters.  (Modified  from  Merkel-Henle,  Anatomie.)  The 
fibers  {A)  are  seen  emerging  from  the  lamina  spiralis,  some  terminating  synaptically 
about  the  inner  hair  cells,  (3),  and  the  remainder  crossing  the  'tunnel  of  Corti'  (5) 
to  terminate  around  the  outer  hair  cells  (6).  The- black  dots  (*)  represent  cross- 
sections  of  the  convolutions  of  the  nerve  fibers. 


human  being.  In  the  vestibule  and  semicircular  canals  of  the  ear  are 
small  hair  cells,  not  serially  arranged,  in  contact  with  which  are  the 
branches  of  nerve  fibrils  from  the  vestibular  branch  of  the  cochlear  nerve. 

A  somewhat  similar  form  of  end  cell  is  in  contact  with  the  terminations 
of  the  gustatory  nerve  fibres  in  the  tongue  (fibers  of  the  9th  or  glosso- 
pharyngeal nerve  and  of  the  lingual  branch  of  the  5th  or  trigeminal 
nerve).  This  is  the  gustatory  cell,  found  in  the  taste  bud  [Fig.  49]. 
Taste  buds  are  irregular  ellipsoid  or  conical  bodies  70-80ju,  in  diameter, 
lying  in  the  epithelium  of  the  mucous  membrane,  most  numerously  in  the 
sides  of  the  annular  groove  in  the  circumvallate  papillae  [Fig.  48]  (prob- 
ably 100  to  150  for  a  single  papilla).  They  are  found  in  smaller  numbers 
in  the  '  foliate  papillae  ',  and  on  the  edges  and  tip  of  the  tongue.  Some- 
times buds  are  found  in  the  '  fungiform  papillae ',  the  soft  palate,  the 


60 


PSYCHOBIOLOGY 


posterior  surface  of  the  epiglottis,  and  occasionally  in  the  lining  of  the 
cheek. 

Taste  buds  are  composed  of  two  kinds  of  cells:  gustatory  and  sup= 
porting  cells.  The  supporting  cells  form  the  enclosing  walls  of  the  bud, 
and  are  also  found  inside  it  between  the  gustatory  cells.  These  latter  are 
in  general  elongated  and  spindle-shaped,  although  found  in  various  wedge 
and  other  shapes.  At  the  outer  edge  of  the  bud  is  a  small  opening;  the 
inner  taste  pore,  from  which  a  pore=canal  leads  between  the  epithelial 
cells  to  the  surface  of  the  epithelium,  terminating  in  the  outer  taste  pore^ 


Taste    />'' 
Buds    C- 


% 


Fig.  48.  Vertical  cross-section  of  a 
-papilla  circumvallata,  showing  taste- 
buds  in  the  side.  Magnified  probably 
100  diameters.  ( Merkel-Henle,  Ana- 
tomie.) 


Fig.  49.  Taste-bud,  schematic.  Mag- 
nified probably  500  diameters.  (Mer- 
kel-Henle, Anatomie.) 


The  gustatory  cells  have  stiff  hair-like  processes  (cilia)  extending 
through  the  inner  pore  into  the  pore  canal.  The  number  of  gustatory  cells 
in  a  bud  varies;  sometimes  there  are  only  two  or  three;  sometimes  they 
are  as  numerous  as  the  supporting  cells. 

The  fibers  of  the  glosso-pharyngeal  and  lingual  nerve  form  plexuses 
below  the  epithelium,  and  from  these  two  sets  of  fibers  rise,  one  set  ending 
free,  in  numerous  knobbed  branches  between  the  epithelial  cells,  and  the 
other  set  entering  the  taste  buds,  at  the  bottom,  usually  from  two  to  five 
for  each  bud.  Inside  the  bud  the  fibers  divide,  multiply,  and  the  branches 
end  (usually  with  minute  knobs)  between  the  gustatory  and  supporting 
cells. 

CORPUSCULAR  AND  SPINDLE  ORGANS. 

There  are  several  forms  of  end  Organs  which  may  be  described  as  con- 
nective tissue  corpuscles  or  capsules,  containing  a  core  of  soft  material 


The  Afferent  and  Efferent  Neurons 


61 


Fig.  50.     Longitudinal  section  of  tactile  papilla,  containing  a  Meissner's  corpuscle. 
Magnified  probably  400  diameters.     (Ranvier,  Histologie.) 


Stratum 
conieum 


J 

-Stratum  lucidum 

Stratum 

graimlosum 


-■ .  o 

9.    ~"    Stratum 
1  *  '  -,.r'    mucosum 
*  v*   *• 

Stratum 
;erminativum 


^[r^fe^v.^f^f;^^       ;      ^/     >l*f    cor,,,, 


Nervous 
a  of 


Fig.  51.     Section  through  human  skin,  showing  the  layers  of  the  epidermis  and  the 
papilla  in  the  corium.     Magnified  probably  140  diameters.     (Cunningham,  Anatomy.} 


62 


PSYCHOBIOLOGY 


which  appears  to  be  composed  of  nucleated  protoplasm,  within  which  the 
dendrite  branches  more  or  less  complexly  and  terminates. 


Fig.  52.  Section  through  a  terminal 
corpuscle  (end-bulb  of  Krause),  from 
the  conjunctiva.  Magnified  probably 
500  diameters.  (Barker,  Nervous  Sys- 
tem, after   Dogiel.) 


Fig.  53.  Section  of  a  Pacinian  cor- 
puscle. Magnified  probably  50  diam- 
eters. (Ranvier,  Histologie.)  The 
nerve  fiber,  n,  n,  enters  the  capsule 
through  the  channel  /,  and  has  its  ter- 
minal branches  at  a. 


\  // 


$ 


Fig.    54.     A    tendinous   terminal   organ    ('organ   of   Golgi'),    on    the    'tendon   of 
Achilles'.     Magnified.     (Barker,  Nervous  System,  after  Cicccio.) 


The  Afferent  and  Efferent  Neurons       63- 

Tactile  corpuscles  [Meissner's  corpuscles)  and  end  bulbs  (end  bulbs 
of  Krause,  genital  corpuscles,  etc.)  resemble  enlargements  of  the  fiber, 
cylindrical  or  spheroidal  in  the  end  bulbs,  and  ellipsoidal  in  the  tactile  cor- 
puscles. The  core  contains  several  nucleated  cells.  The  nerve  fiber  may 
wrap  around  the  tactile  corpuscle  several  times  before  entering  it. 

Tactile  corpuscles  [Fig.  50]  occur  in  some  of  the  papillae  of  the  skin 
of  the  hand  and  foot,  and  more  sparingly  in  the  back  of  the  hand,  the 
inner  surface  of  the  forearm,  the  lips,  eyelids,  nipples,  and  external  genital 
organs.  End  bulbs  (other  than  the  genital  corpuscles)  are  often  very 
small,  but  sometimes  are  over  50jU,  in  diameter.  Genital  corpuscles  are 
from  20 ja  to  350ft  in  diameter. 

Tactile  corpuscles  are  always  within  the  connective  tissue  layer  of  the 
skin,  the  corium  [Fig.  51].  They  are  from  80/a  to  150 /a  in  length,  and 
%  as  broad. 


Fig.  55.  Middle  third  of  a  '  terminal  placque '  in  a  muscle  spindle  of  a  cat. 
Highly  magnified.  (Barker,  Nervous  System,  after  Ruffini.)  At  N  are  seen  three- 
nerve  fibers,  whose  branches  compose  the  rings  {A),  spirals  (S),  and  branched  pro- 
cesses (F)  making  up  the  '  placque '.  Three  muscle  fibers  are  shown,  enclosed  in. 
the  sheath  C ,  C. 

End  bulbs  [Fig.  52]  are  found  in  the  conjunctiva  in  the  corium  in  or 
below  the  papillae  of  the  lips  and  tongue,  in  serous  membranes,  in  tendons,, 
aponeuroses,  the  epineurium  of  nerve  trunks,  in  the  neighborhood  of  the 
joints  and  in  the  corium  of  the  modified  skin  covering  the  external  genital 
organs. 

Pacinian  corpuscles  {Vater-Pacinian  corpuscles :  Golgi-Mazzonni  cor- 
puscles) are  larger  than  end  bulbs,  and  formed  with  many  concentric 
layers  of  connective  tissue,  like  layers  of  an  onion.  A  single  medullated 
nerve  fibre  enters  the  corpuscle,  losing  its  medullary  sheath  and  branching 
within  [Fig.  53]. 


64 


PSYCHOBIOLOGY 


Pacinian  corpuscles  are  found  in  the  subcutaneous  tissues,  in  the  corium, 
the  hands  and  feet,  in  the  periosteum  of  some  bones, — in  the  neighborhood 
of  tendons  and  ligaments,  and  in  the  connective  tissues  at  the  back  of  the 
abdomen.     They  are  relatively  large,  being  sometimes  1.5mm.  in  length. 

TENDON  AND   MUSCLE   SPINDLES. 

A  special  form  of  ending,  the  organ  of  Golgi,  is  found  in  the  tendons 
near  the  point  of  attachment  of  muscle  fibers.  The  tendon  bundle  here 
becomes  enlarged,  and  splits  into  a  number  (8  to  20)  of  smaller  fasciculi. 


Fig.  56.  A  Ruffini's  nerve  ending.  Highly  magnified.  (Barker,  Nervous  System, 
after  Ruffini.)  The  ramifications  of  the  nerve  fiber  gH  are  enclosed  in  the  connec- 
tive-tissue capsule  L. 


The  Afferent  and  Efferent  Neurons 


65 


One  or  more  nerve  fibers  penetrate  between  these  fasciculi,  losing  their 
medullary  sheathes,  and  arborizing  there  [Fig.  54].  The  whole  enlarge- 
ment is  enclosed  in  a  connective  tissue  capsule  continuous  with  the  areolar 
tissue  between  the  tendon  bundles.  Tendon  spindles  are  from  1  to  1.5  mm. 
in  length. 

A  somewhat  similar  mode  of  termination  occurs  in  the  muscle  spindle. 
This  consists  of  a  connective  tissue  sheath  enclosing  a  bundle  of  two  to 
twenty  or  more  muscle  fibers  (an  'intrafusal  bundle')  which  are  smaller 


-Oblique  section  through 
Papilla  of  hair        a  Pacinian  corpuscle 

Fig.  57.     Schematic  vertical  section  through  the  skin,  showing  a  hair,  sweat  glands, 
and  sebaceous  glands.      (Cunningham,  Anatomy.) 

than  the  surrounding  fibers  and  more  like  embryonic  fibers.  The  sub- 
divisions of  some  of  the  nerve  fibers  which  enter  the  spindle  wrap  them- 
selves spirally  about  the  muscle  fibers.  The  branches  of  others  terminate 
in  plate-like  expansions  applied  to  the  fibers  [Fig.  55]. 


66 


PSYCHOBIOLOGY 


Muscle  spindles  occur  in  the  muscles  generally,  except  in  the  tongue, 
where  none  have  yet  been  discovered.  Evans  has  found  them  to  be  espec- 
ially numerous  in  the  eye-muscles.  They  are  from  1  to  5  or  more  mm.  in 
length,  and  1  to  3  mm.  in  the  broadest  parts. 

RUFFINl'S    ENDINGS 

These  are  end  organs  lying  at  the  junction  of  the  corium  and  the  sub- 
cutaneous connective  tissue  of  the  fingers  and  toes,  and  also  deeper  in  the 
subcutaneous  tissues.  They  resemble  the  spindle  somewhat,  having  a  con- 
nective tissue  sheath  within  which  the  nerve  fibers  (in  some  cases  two 
fibers)  ramify  [Fig.  56]. 

ENDINGS  IN  HAIR  FOLLICLES. 

In  every  hair  follicle  one  or  more  afferent  dendrites  terminate.  The 
hair  follicle  is  a  pocket  in  the  skin,  and  hence  consists  of  an  outer  sheath, 


Fig.  58.     Nerve  endings  about  a  large  hair  of  a  dog.      (Barker,  Nervous  System, 
after  Bonnet.) 

the  theca,  continuous  with  the  corium,  and  an  outer  root  sheath,  con- 
tinuous with  the  deeper  layers  of  the  epidermis   (i.  e.,  with  the  stratum 


The  Afferent  and  Efferent  Neurons 


67 


germinativum  and  stratum  rrmcosum).  Within  them  is  an  inner  root 
sheath. 

The  theca  is  in  three  layers :  an  outer  layer  of  loose  tissue,  a  middle 
layer  of  compact  circular  bundles,  and  a  thin  inner  layer,  called  the 
glassy  layer,  (hyaline  membrane  or  vitreous  membrane). 

The  nerve  fibre  or  fibres  entering  a  follicle  penetrate  at  about  the 
median  level  to  the  glassy  layer,  where  each  forms  two  branches  which 
almost  completely  encircle  the  hair,  and  on  the  opposite  side  arborize 
[Fig.  58].  The  nerve  terminals  very  rarely  penetrate  through  the  glassy 
layer. 

Since  hairs  are  present  over  the  entire  body  except  the  palms  of  the 
hands,  the  soles  of  the  feet,  the  dorsal  surface  of  the  terminal  segments 
of  the  fingers  and  toes,  and  certain  parts  of  the  genital  organs,  it  is 
apparent  that  they  constitute  an  important  group  of  receptor  organs. 

AFFERENT  NEURONS  OF  THE  OLFACTORY  MEMBRANE. 

The  cell  bodies  of  the  olfactory  afferent  neurons  are  in  the  olfactory 


Fig.  59.     Olfactory  cells  and  sustentacular  cells,  schematic.     Magnified  about  800 
diameters.     (Merkel-Henle,  Anatomie.)     0,  olfactory  cell;  s,  sustentacular  cell. 

epithelium.  Outwardly,  the  cell  has  a  short  slender  process  (correspond- 
ing to  a  dendrite)  ending  in  a  hemispherical  knob  that  projects  slightly 
beyond  the  general  epithelial  surface,  and  bears  from  six  to  eight  stiff 


68 


PSYCHOBIOLOGY 


cilia   (the  olfactory  hairs)    [Fig.   59].     From  the  other,  deeper,  end  the 
cell  sends  an  axon,  which  passes  in  through  orifices  in  the  bone    (the 


Fig.  60.  Schematic  representation  of  some  of  the  principal  neurons  of  the  olfac- 
tory conduction  paths.     (Barker,  Nervous  System.) 

'  cribriform  plate  '  of  the  '  ethnoid  '  bone)   and  arborizes  in  the  olfactory 
glomeruli  of  the  olfactory  bulb  (bulbus  olfactoriits)    [Fig.  60]. 

AFFERENT   CHAINS  OF  THE  OPTIC   NEURONS. 

In  the  eye,  we  have  to  consider  not  a  single  afferent  neuron,  but  a  series 
of  three,  forming  an  afferent  chain  [Fig.  61].  The  outermost  neurons 
are  specialized  receptor  cells  of  two  types :  rod  cells  and  cone  cells. 
The  rods  and  the  cones  are  the  outermost  portions  of  these  cells,  and 
are  the  structures  which  receive  stimulation  from  the  light  waves.  The 
rods  in  the  human  retina  are  approximately  60/a  in  length  and  2ja  in 
diameter.  The  cones  are  about  35/a  in  length,  the  '  outer  segment '  being 
approximately  of  the  diameter  of  a  rod,  and  the  '  inner  segment '  about 
7//.  in  diameter. 

The  rods  and  cones  are  packed  closely  together  with  their  long  axes 
perpendicular  to  the  surface  of  the  retina.  In  the  fovea  centralis  there 
are  only  cones :  in  the  macula  lutea  surrounding  the  fovea,  each  cone  is 
encircled  by  a  row  of  rods.  Farther  away  from  the  fovea  (z.  <?.,  in  the 
medial  and  peripheral  zones  of  the  retina)  the  rods  are  relatively  more 
numerous,  adjacent  cones  being  separated  by  2,  3,  4  or  more  rods. 


The  Afferent  and  Efferent  Neurons 


69 


The  rod  and  cone  may  be  considered  as  special  forms  of  dendrites. 
The  rod  is  connected  with  the  cell  body  by  a  slender  fiber,  whereas  the 
inner  segment  of  the  cone  is  practically  a  continuation  of  the  cell  body. 

From  the  inner  side  of  the  cell  body  a  short  axon  proceeds,  which, 
in  the  case  of  the  cone  cell,  has  branches  in  contact  with  the  dendrites  of 
the  bipolar  cells  in  the  '  inner  nuclear  layer '  of  the  retina.  The  rod 
cell  axons  end  in  a  single  knob,  in  contact  with  the  dendritic  branches 


Fig.  6i.  Schematic  representation  of  trie  sensory  apparatus  in  the  retina  of  the 
human  eye.  (Merkel-Henle,  Anatomie.)  i,  Layer  of  pigment  cells  next  to  the  cho- 
roid. 2,  Processes  of  the  pigment  cells.  3,  Rods.  4,  Bodies  of  rod-cells.  5,  Cones. 
6,  Axons  of  cone  cells.  7,  Cone-bipolar  cells.  8,  9,  Ganglion  cells.  10,  Optic  nerve 
fibers  (axons  of  ganlion  cells).  11,  12,  Horizontal  cells.  13,  14,  15,  16,  Cells  of  dif- 
ferent types;  functions  unknown.  17,  Fibers  (axons  probably)  of  cells  having  bodies 
in  the  brain.  18,  Neuroglia  cells.  19,  Radial  fiber  (Miiller's  fiber;  part  of  the  susten- 
tacular  syncytial  framework  of  modified  neuroglia. 


of  the  bipolar  cells.  Each  cone  cell  axon  has  synaptic  connection  with 
one  cone-bipolar  cell.  The  axons  of  several  rod  cells  may  arborize  with 
the  dendrites  of  a  single  rod-bipolar.  In  this  layer  there  are  also  hori= 
zontal  cells  whose  axon  and  dendrite  branches  are  in  contact  with  the 
terminations  of  the  rod  and  cone  cell  dendrites.  These  horizontal  cells 
are  therefore  interconnecting  cells  (so-called  association  cells). 

The    bipolar    cells    send    axons    inward    to    varying    distances,    which 


70  PSYCHOBIOLOGY 

touch  the  dendritic  terminations  of  the  ganglion  cells  in  the  next  to  the 
innermost  layer  of  the  retina,  or  possibly  in  some  cases  touch  the 
ganglion  cell  bodies.  These  axons  of  these  ganglion  cells  run  to  the 
blind  spot  or  disc  of  the  eye,  and  from  thence  in  the  optic  nerve,  to  the 
chiasm  (c  Mas  ma  opticum),  [Fig.  67],  where  the  fibers  for  the  right  half 
of  each  retina  pass  to  the  right  optic  tract,  and  the  fibers  for  the  left  half 
to  the  left  optic  tract.  In  each  optic  tract  some  of  the  axons  run  to  the 
external  geniculate  body  {corpus  geniculatum  laterale),  [Fig.  69], 
and  some  to  the  internal  geniculate  body  {cor p.  gen.  mediate),  on 
the  same  side,  arborizing  there  in  contact  with  interconnecting  neurons, 
whose  fibers  continue  in  the  optic  tract  to  the  '  occipital  lobes  '  of  the 
cerebral  cortex,  or  run  to  other  ganglia  in  the  mid-brain. 

THE  EFFERENT  NEURONS 

The  efferent  neurons  of  the  spinal  system  have  their  cell  bodies  in  the 
gray  matter  of  the  cord,  the  axons  leaving  the  cord  on  the  anterior  side 
(forming  therefore  the  anterior  roots),  and  joining  the  fibers  of  the 


Fig.  62.     End-plates  (motor  nerve  endings)  in  striped  muscle  of  a  rat.     Magnified 
170  diameters.      (Szymonowicz,  Histologie.) 

'  posterior  roots '  to  make  the  complete  spinal  nerve  just  beyond  the 
'  spinal  ganglion  '.  Some  of  these  fibers  (somatic  fibers)  run  in  the 
nerve  and  its  branches  directly  to  endings  in  striped  muscles.  Others, 
(visceral  or  splanchnic  fibers)  leave  the  nerve  beyond  the  spinal  ganglion 


The  Afferent  and  Efferent  Neurons 


71 


and  run  over  the  white  rami  communicantes  to  the  ganglia  of  the 
sympathetic  chain  (the  lateral  ganglia).  Certain  of  the  splanchnic  fibers 
arborize  each  around  a  number  of  cells  in  the  sympathetic  ganglia;  others 
run  through  these  ganglia  to  the  collateral  ganglia,  arborizing  in  the 
same  way  there.  The  axons  from  lateral  and  collateral  ganglia  run  to 
the  smooth  muscle  tissue  or  to  glands.  Thus,  the  current  leaving  the 
spinal  cord  over  a  single  splanchnic  fiber  is  distributed  finally  to  a 
number  of  axons.  Many  of  the  axons  from  the  lateral  ganglia  return 
to  the  spinal  nerve  over  the  gray  rami  [Figs.  72,  73  and  74]. 


mm 

nil?  iB  i 

gin  i  si  fa 

IIS 


Fig.  63.  End  plate  of  a  lizard.  Enlarged  550  diameters.  (Schafer,  Microscopic 
Anatomy,  after  Kiihne.)  n,  Nerve  fiber,  o,  Terminal  ramifications,  m,  Matrix 
(clear  substance  surrounding  the  ramifications),  b,  Granular  bed,  or  sole,  of  the  end 
organ. 

What  has  been  said  about  spinal  afferent  neurons  applies  in  general 
to  the  efferent  neurons  of  the  cranial  nerves,  whose  cells  lie  in  the  nuclei 
of  the  medulla,  pons  and  mid-brain. 

As  the  fiber  approaches  its  termination  in  voluntary  (striated)  muscle, 
it  branches  repeatedly,  each  fiber  thus  coming  into  relation  to  a  number 
of  muscle  fibers.  When  a  branch  reaches  a  muscle  fiber,  the  medullary 
sheath    ceases    abruptly,    the   neurilemma   becomes    continuous    with    the 


72  PSYCHOBIOLOGY 

sarcolemma  beneath  which  the  fiber  branch  terminates  in  the  end  plate, 

oval,  from  40  to  60/x.  in  its  longest  diameter.  Occasionally  a  branch 
terminates  in  two  end  plates.  In  this  end  plate,  of  nucleated  protoplasm, 
the  fiber  arborizes,  with  enlarged  ends  [Figs.  62  and  63]. 

In  smooth  and  cardiac  muscle,  the  fibers  of  the  terminal  ganglia 
branch  and  pass  between  the  fibrils,  ending  in  enlarged  outlets  and  knobs, 
in  a  fashion  much  simpler  than  that  in  the  case  of  the  striated  muscle 
[Fig.  64]. 

A. 


*^8s»_ 


Fig.  64.  Efferent  nerve  endings  on  smooth  muscle  fibers.  Magnified  probably  300 
diameters.     (Barker,  Nervous  System,  after  Huber  and  De  Witt.) 

Efferent  nerve  endings  in  glands  are  somewhat  similar  to  the  endings 
in  smooth  muscle,  the  branches  of  the  fiber  penetrating  between  gland 
cells  and  having  spheroidal  varicosities  on  their  terminal  segments.  In 
some  cases  the  final  branches  seem  to  enter  the  cytoplasm  of  the  gland 
cells. 

REFERENCES  ON  AFFERENT  AND  EFFERENT  NEURONS. 

Lewis  and  Stohr,  Histology,  Pt.  I,  §  II,  The  Central  Nervous  System. 

Sherrington,  The  Spinal  Cord.     In  Schafer's  Physiology. 

Schafer,  Microscopic  Anatomy,  §  The  Structure  of  the  Tissues,  Sub-§  The  Tissues  of 

the  Nervous  System. 
Barker,  The  Nervous  System,  Chapters  XXVI-XXXVIII,  LV-LVI. 
Bailey,  Histology,  Chapter  XII. 


CHAPTER  VI. 

THE  GROSS  RELATIONS  OF  NERVES,  SPINAL  CORD,  BRAIN  AND  OTHER  GANGLIA. 

In  speaking  of  the  human  brain  stem,  the  term  '  anterior  '  means  upper 
and  '  posterior  '  means  lower.  These  terms  are  used  in  this  way  because 
of  the  usual  reference  of  the  brain  structures  to  the  medullary  tube  from 
which  the  cerebro-spinal  system  develops.  The  brain  develops  from  the 
anterior  portion  of  this  tube;  the  upper  part  of  the  brain  stem  (the  fore- 
brain)  develops  from  the  most  anterior  portion.  Moreover,  in  the  quad- 
rupeds, in  which  the  cerebro-spinal  axis  is  horizontal,  the  '  anterior  '  por- 
tions are  really  anterior  to  the  '  posterior  '. 


Sulcus  centralis  (Rolandi) 
Salens  praecentralis. 
Sulcus  frontalis  superior 
Sulcus  frontalis  inferior. 


Sulcus  interparietal! s 


Fissnra  cerebri 
lateralis  (Sylvii) 


Ramus  anterior 
irorizon  talis 

Ramus  anterior, 
ascendens 

Ramus  posterio 
Sulcus  temporalis  medius 


Sulcus  temporalis  superior 


Fig.  65.  Brain  viewed  from  the  left  side,  showing  lateral  surface  of  the  left  cere- 
bral hemisphere.      (Toldt,  Anatomischer  Atlas.)     One-half  normal  size. 

The  spinal  cord  [Figs.  29,  37]  extends  from  about  the  '  small '  of  the 
back  into  the  skull,  where  it  ends,  at  about  the  level  of  the  ears,  in  the 
enlargement  called  the  medulla  oblongata  (or  myelencephalon) ,  [Figs. 
65-70],  which  is  about  an  inch  in  length.  Above  the  medulla  on  the  front 
is  a  fibrous  band,  the  pons,  running  horizontally  across  and  into  the 
cerebellum,  which  lies  above  and  behind  the  medulla.  (Pons  and  cere- 
bellum together  constitute  the  met ence photon).  These  are  the  three  divis- 
ions of  the  hind=brain  (or  rhombencephalon) . 


74 


PSYCHOBIOLOGY 


Above  the  medulla  and  continuous  with  it  is  the  mid=brain  (or  mesen- 
cephalon), about  three  quarters  of  an  inch  in  length,  on  the  back  of  which 
are  the  protuberances  known  as  the  corpora  quadrigemina,  and  on  the 
front  the  beginning  of  division  into  right  and  left  portions,  the  crura 
(singular  crus).  This  division  becomes  complete  in  the  diencephalon, 
forming  the  two  thalami,  upon  which  are  superposed  the  corpora  striata 
[Fig.  70].  From  the  upper  part  of  the  thalami  grow  the  cerebral  hemis= 
pheres  or  cerebri    (or  telencephalon),  which  are  the  largest  and  most 


Ventrical 

Tela  chorioidea  ventriculi  ten! 
Corpus  pineale 
Fissura  transversa  cerebri 
Aquaeductus  cerebri  (Sylvii) 
Lamina  quadrigemina 
Velum  medullare 
anterius 
Ventriculus 
quartus 


Massa  intermedia 


nterventriculare  (Monroi) 
mna  fornicis 

Septum  pellucidum 

Lamina  rostralis 


*/. 

inissura  anterior 
"~-~.  Lamina  terminalis 
v  Chiasma  opticum 
Infundibulum 
X  Hypophysis 
\Tuber  cinereum 
Corpus  mamillare 

Pons  (Varoli) 

Fig.  66.  Medial  sagittal  cross-section  of  brain,  showing  medial  surface  of  left 
hemisphere,  one-half  normal  size.     (Toldt,  Anatomischer  Atlas.) 

conspicuous  parts  of  the  brain,  folding  over  and  concealing  nearly  all  of 
the  mid-brain.  Diencephalon  and  telencephalon  together  constitute  the 
fore-brain   (or  prosencephalon)    [Fig.  28]. 

The  pons,  medulla,  mid-brain  and  thalami  together  make  up  the  brain= 
stem. 

It  is  important  to  distinguish  certain  parts  and  cavities  of  the  brain, 
even  if  we  make  but  a  simple  study  of  its  structure,  nervous  connections 
and  probable  functions. 

In  all  of  these  parts  we  find  nerve  cells,  and  also  their  axons  and  den- 
drites. The  cell  bodies  are  in  general  collected  in  the  cortex  or  external 
portions  of  the  cerebellum  and  cerebrum, -and  in  groups  scattered  through 
the  brain  stem.  These  groups  are  called  nuclei,  and  most  of  these  nuclei 
have  received  distinctive  names. 


Nerves,  Spinal  Cord,  Brain  and  Other  Ganglia         75 

In  addition  to  the  substances  of  the  brain,  it  is  important  to  distinguish 
the  cavities  or  ventricles,  since  it  is  often  convenient  to  refer  to  the 
position  of  a  nucleus  or  other. detail  as  being  in  the  wall  of  a  certain  ven- 
tricle, although  the  cavity  may  be  relatively  very  small  as  compared  with 


Plexus  chorioideus  venlriculi  terlii 
Ventriculus  tertius 
Thalamus  (Fades  medialis) 
Sulcus  hypothalami cus  (Monroi^ 
Aditus  ad  aquaeduclum  cerebri 
Commissura  posterior  (cerebri)    [ 
Commissura  habenularum, 
Recessus  pinealisi 
Recessus  suprapinealis.     \ 
Corpus  pineale  > 


V.  cerebri  magna,  (Galeni) 

Lamina  quadrigemina 
Aquaeductus  cerebri 


Decussatio  brachii  conjunctivi 
Velum  medullare  anterius 


Fasciculus  longitud 
nalis  medialis 


Vermis 


Ventriculus quartus  ___/^_rSi_    ■*KgL-~-~- 

Fastigium_7^^^i7 


Hemisphaerium 

cerebelli  j//         / 

Laminae  medulla'res?    ■     / 

Plexus  chorioideus  ventriculi  quarti' 

Calamus  scriptorius' 


Fig.  67. 
Atlas.) 


Tela  chorioidea  ventriculi  tertii 

Corpus  for-nicis 

Foramen  interventriculare 

(Monroi) 

Septum  pellucidum 

Rostrum  corporis 
callosi 

Gyrus  sub- 
call  osus 

Commissura 
anterior 

„. Hypothalamus 

^Pw^J#"/' ._  Lamina  terminalis 
.  ^asr..  Recessus  opticus 

Chiasma  opticum 

■—  Recessus  infundibuli 
Infundibulum 

l__  Hypophysis  (Lobus  anterior 

und  posterior) 

v  Corpus  mamillare 

Nervus  oculomotorius 

\      \     v    Recessus  anterior 

i    \      \     'Fossa  interpeduncularis  (Tarini) 

\    i      'Sulcus  n.  oculomotorii 

\     \Recessus  posterior 

Pons  (Varoli)   (Fibrae  superficiales) 

Fasciculi  longitudinales  (pyramidales) 

i       1       Foramen  caecum 

\        Pyramis  medullae  oblongatae 
1/     \ 
't     'Raphe  medullae  oblongatae 

Decussatio  pyramidum 

1  Medulla  spinalis 

Medial    section   of   brain    stem    and   cerebellum.      (Toldt,    Anatomischer 


the  thickness  of  the  so-called  '  walls  '.  In  another  respect  the  cavities 
are  important,  since  the  brain  and  cord  are  formed  from  a  single  tube, 
which,  in  the  foetus,  has  a  relatively  large  bore  and  thin  walls,  the  brain 
and  cord  proper  forming  by  actual  thickening  of  these  walls. 


76 


PSYCHOBIOLOGY 


The  cavities  of  the  brain  are  as  follows: 

First  and  second  ventricles  [yentriculi  laterales)  :  hollows  within 
the  right  and  left  cerebral  hemispheres  respectively  [Fig.  70].  Third 
ventricle  [ventriculus  tertius),  lying  between  the  optic  thalami,  the  inner 
surfaces  of  which  form  its  side  walls  [Fig.  66].  The  'floor'  or  ventral 
boundary  is  formed  by  the  tuber  cinereum,  the  corpora  mamillaria,  the 
gray  matter  of  the  locus  perforatus  posticus,  and  the  tegmenta  of  the 


Bulbus  olfactorius 
Tract  us  olfactorius 
i  opticum 
N.  opticus 


Fissura  longitudinals' cerebri 
;  olfactorius 


Hypophysis 


Trigonum  olfaclo 
Tractus  opt: 

N  oculomotorius 


Fissura  cerebri  lateralis 
(Sylvii) 

Polus  temporalis 

,. Substantia  perforata 
anterior 

Infundibulura 
.  Tuber  cinereum 
'Corpus  roamillare 


■  Fossa 

cula 


nterpedu 
I  (Taiim 


N.  accessorius 


N.  hypoglossus 

N.  spinalis  I 


-  Pedunculus 
cerebri 

Pons  (Varoli) 

Flocculus 

Plexus  chorioideus 
triculi  quarti 

Foramen  caecum 


Hemisphaerium  cerebelli 
Medulla  oblongata 
Decussatio  pyramidum 

Medulla  spinalis 
Polus  occipitalis 


Fig.  68.  Brain  viewed  from  the  front  and  below,  one-half  natural  size.  (Toldt, 
Anatomischer  Atlas,) 

crura  cerebri.  The  '  frontal  wall '  or  upper  boundary  is  the  lamina 
cinerea,  and  the  '  roof  '  or  dorsal  boundary,  an  epithelial  layer  continuous 
with  the  epithelium  lining  the  chamber.  At  the  'anterior'  (or  upper) 
end  the  third  ventricle  communicates  with  the  first  and  second  ventricles, 
and  at  the  '  posterior '  end  it  communicates  through  the  aqueduct  of 
Sylvius  (aque 'ductus  cerebri),  a  very  small  tubular  aperture  through  the 
mid-brain,  with  the  fourth  ventricle   (ventriculus  quartus). 

The  fourth  ventricle  is  the  cavity  under  the  cerebellum  (the  hind- 
brain),  its  ventral  wall  being  the  medulla,  and  its  dorsal  wall,  a  thin 
epithelial  membrane. 


Nerves,  Spinal  Coed,  Brain  and  Other  Ganglia         77 

The  aqueduct  of  Sylvius  and  the  third  and  fourth  ventricles  (except 
for  the  dorsal  walls  or  '  roofs  '  of  these  latter)  are  surrounded  by  layers 
of  gray  matter,  forming  the  nuclei  of  the  third,  fourth,  sixth,  twelfth  and 
the  cranial  nerves,  and  the  secondary  nuclei  of  the  eighth  nerve,  and  of 
the  afferent  portions  of  the  i.inth,  tenth  and  eleventh  nerves.  The  nuclei 
for  the  fifth  nerve  and  the  efferent  portions  of  the  ninth,  tenth  and 
eleventh  lie  in  the  immediate  neighborhood. 

GROSS  DETAILS   OF  THE   BRAIN. 

Certain  details  of  the  external  appearance  of  the  medulla,  mid-brain 
and  fore-brain  are  of  importance  both  as  indicating  structural  details  and 
as  points  of  topographical  reference. 

On  the  ventral  side  of  the  medulla  the  olives  (olivae),  the  pyramids 
(pyramis),  and  the  decussation  of  the  pyramids  (decussatio  pyra- 
midum),  are  noticeable  [Fig.  68].  On  the  dorsal  side  the  cuneate 
tubercules,  the  clava,  the  funiculus  gracilis,  and  the  funiculus  cune= 
atus  appear  [Fig.  69A].  Above  the  medulla  the  pons  appears  on  the  ven- 
tral side,  and  the  cerebellum  on  the  dorsal,  with  middle  and  superior 
cerebellar  peduncles  [brachia  p otitis  and  brachia  conjunctiva)  joining 
it  to  the  ends  of  the  pons  and  to  the  brain  stem  behind  the  pons  [Figs. 
68  and  69].  | 

Conspicuous  on  the  floor  and  side  walls  of  the  fourth  ventricle  (between 
the  stem  and  the  cerebellum)  are  the  striae  acusticae  (or  stria  medu- 
lares)  crossing  the  area  acustica,  the  eminentia  teres  (colliculus 
facialis)  and  the  beginning  of  the  Sylvian  aqueduct  (Aqueductus 
cerebri)   [Fig.  69A]. 

The  chief  features  of  the  mid-brain  are  the  corpora  quardigemina  on 
the  dorsal  surface  (two  pair,  '  inferior  '  and  '  superior  '  [Fig.  69 A]  and  the 
pineal  body  (corpus  pineale)  above  them  [Fig.  66].  The  ventral  aspect 
shows  the  crura  (singular  crus)  cerebri  [peduncidi  cerebri)  with  the 
sulcus  oculomotorius  separating  them,  and  lying  in  front  of  the  upper 
part  of  these,  the  corpora  mamillaria  [Fig.  67]. 

Still  farther  forward  is  the  protuberance  known  as  the  tuber  cine= 
reum  from  which  the  infundibulum  extends  down  to  the  pituitary 
body  {hypophysis)  [Fig.  66]. 

The  mid-brain  between  the  crura  and  the  corpora  quadrigemina  is  called 
the  tegmentum.    The  ventral  portions  of  the  crura  are  called  crustae. 

The  crustae  of  the  crura  are  separate  in  the  mid-brain.  Above,  the 
tegmentum  also  divides,  and  each  half  of  the  brain  stem  enters  a  thala= 
mus.  The  thalami  are  ovoid  masses  of  gray  matter,  divided  into  three 
nuclei,  the  nucleus  anterior  (or  dorsal  nucleus),  the  nucleus  medialis  and 
the  nucleus  lateralis. 


78 


PSYCHOBIOLOGY 


Frenulum 
Valve  of  Vieussens 


Superior  peduncle  of 
the  cerebellum 

Middle  peduncle  of 
the  cerebellum 


Striae  acusticse 
Area  acusticse 
Trigonum  vagi 

Cuneate  tubercle 
Funiculus  gracilis 


Taenia  thalami 


Pineal  body 

Superior  quadri- 
geminal  body 

Inferior  quadri- 
geminal  body 


Crus  cerebri 

Pontine  part  of  floor 
of  ventricle  IV. 

Eminentia  teres 

Fovea  superior 

Restiform  body 
Trigonum  hypoglossi 
Clava 

Rolandic  tubercle 
Funiculus  cuneatus 


Fig.  69A.     Floor  of  fourth  ventricle,  and  rear  of  medulla  and  mesencephalon  of 
full-time  foetus.     (Cunningham,  Anatomy.) 


Optic  tract 

rus  cerebri 
Corpus  geniculatum 
externum 


•-j  exrerni 
-r^-Pulvihi 


mar 
Corpus  geniculatum 
internum 

Superior  brachium 
Inferior  brachium 
Inferior  quadrigeminal  body 
Lateral  fillet 

uperior  cerebellar  peduncle 
Taenia  pontis 

Middle  peduncle  of 
cerebellum 


.Restiform  body 
Ligula  bounding  lateral 
Tecess  of  ventricle  IV 
Olivary  emiuence 

■Arcuate  fibres  (anterior  superficial) 
"lava, 

iuneate  tubercle 
Rolandic  tubercle 
Lateral  district  of  medulla 

Anterior  column  of  cord 


Fig.   69B.     Brain  stem  of  full-time   foetus  viewed   from   the  left.      (Cunningham, 
Anatomy.) 


Nerves,  Spinal  Cord,  Brain  and  Other  Ganglia         79 

Laterally  to  each  thalamus  are  two  nuclei  {nucleus  caudatus  and  nucleus 
lenticularis) ,  which  together  form  the  corpus  striatum.  Between  these 
two  nuclei  a  fan-shaped  mass  of  nerve  fibers  (the  internal  capsule)  runs 
up  from  the  crusta.  Lying  behind  the  corpus  striatus  is  another  sheet  of 
fibers;  the  external  capsule   [Fig.  70]. 

The  dorsal  projection  of  the  thalamus  is  the  pulvinar.  Lying  below 
and  outwardly  to  the  pulvinar  are  the  geniculate  bodies  (two  on  each 
side),  the  external  or  lateral  and  the  internal  or  medial  [Fig.  69]. 

Above  the  thalami  are  the  two  hemispheres  of  the  cerebrum  which  are 
spread  out  over  and  behind  the  thalami  and  the  mid-brain.  The  hemis- 
pheres may  be  considered  as  ganglia,  or  groups  of  ganglia,  the  cells  of 
which  are  in  the  outwardly  lying  portions  (the  cortex). 

Running  between  the  hemispheres  is  a  broad  band  of  fibers,  the  corpus 
callosum,  and  two  other  smaller  bundles,  the  anterior  and  posterior 
commissures.  (Many  bundles  of  association  fibers  connect  the  various 
lobes  of  the  same  hemispheres). 

Growing  forward  from  the  under  side  of  each  hemisphere  near  the 
brain  stem  is  a  long  process,  the  olfactory  tract  {tractus  olfactorius)  at 
the  end  of  which  is  the  enlargement  called  the  olfactory  bulb  [Fig.  68]. 

THE   COLUMNS  AND  TRACTS  OF  THE  SPINAL  CORD. 

In  the  '  white  matter  '  of  the  cord  three  pair  of  columns  are  distin- 
guished for  convenience  of  reference  [Fig.  41]. 

1.  The  posterior  columns,  lying  between  the  'posterior  horns'  of 
the  gray  matter,  and  separated  by  the  posterior  fissure  of  the  cord. 

2.  The  lateral  columns,  included  between  the  anterior  and  posterior 
horn  on  each  side. 

3.  The  anterior  columns,  included  between  the  anterior  horns,  and 
separated  by  the  anterior  fissure. 

In  the  columns  of  the  cord  several  distinct  groups  of  fibers  are  dis- 
tinguishable. The  most  important  of  these  are  as  follows :  the  enumera- 
tion beginning  at  the  dorsal  fissure  [Fig.  71]. 

I.  Ascending  {i.  e.,  conducting  towards  the  brain),  (a)  The  greater 
part  of  the  posterior  columns;  specifically,  the  column  of  Burdach 
{postero-lateral  tract  or  fasciculus  cuneatus),  and  the  column  of  Goli 
(postero-mesial  tract  or  fasciculus  gracilis).  (b)  Lissauer's  tract, 
lying  at  about  the  apex  of  the  posterior  horn,  (c)  The  direct  cerebellar 
tract  {dorsal  cerebellar  tract;  fasciculus  cerebello- spinalis ;  Flechsig's 
tract),  and  the  anterolateral  ascending  tract  {fasciculus  antero-later- 
alis  super ficialis ;  Gower's  tract),  occupying  the  superficial  portion  of  the 
lateral  column. 


80 


PSYCHOBIOLOGY 


Fissura  longimdinalis  cerebri 
Radiatio  corporis  callosi  K 


Septum  pellucidum 


Plexus  chorio 
ideus  ventriculi  N 
lateralis 


Corona  radiata.    »Wm 

Columna 
fornicis 
Plexus  chorio- 
ideus  ventriculi  ""V"--.^  f 

tertii     . 
Capsula  interna 

Thalamus 


;  Gyrus  frontalis  superior 

^Truncus  corporis  callosi 

Cornu  anterius  ventriculi 
lateralis 
/  Caput  nuclei  caudati 


Radiatio  cor- 
poris striati 

Putamen 


Ventnculus.  .-,  - -""-tip 

tertius  I .  "  , 

Fossa  inter- { 

peduncularis     ■      ' 

(Tarini)  :  '-'  | 

Cornu  inferius .  -. 
ventriculi  i 
lateralis  II 


Pedunculus 
cerebri 


*^0 

F~  —  Globus  pallidus 


Tractus  opticus 


Brachium  pontis 

Fasciculi  longitudi-^" 
nales  (pyramidales 
pontis 

Facies  inferior  cerebelli 
Fibrae  pontis  superficiales'' 

Pyramis  medullae  oblongatae  ■' 


N.  trigeminus 


Nn.  facialis  und 
acusticus 

Flocculus 


N.  glossopharyngeus 
N.  vagus 
Nucleus  olivaris  inferior 


**■  Decussatio  pyramidum 
Fig.  70.     Coronal  section  of  brain.     Normal  size.     (Toldt,  Anatomischer  Atlas.) 


Nerves,  Spinal  Cord,  Brain  and  Other  Ganglia         81 

The  fibers  in  the  column  of  Goll  and  Burdach  and  Lissauer's  tract  are 
axons  of  the  posterior  roots,  i.  e.,  whose  cell-bodies  are  in  the  spinal 
ganglia.  The  fibers  in  the  direct  cerebellar  tract  have  their  cell-bodies  in 
Clark's  Column;  a  column  of  cells  in  the  posterior  horn.  The  location 
of  the  cell-bodies  of  the  fibers  in  Gower's  tract  have  not  been  definitely 
identified. 

II.  Descending,  (a)  The  septo=marginal  tract,  lying  in  the  edge 
of  the  posterior  column  close  to  the  dorsal  fissure,  (b)  The  comma 
tract,  lying  within  Burdach's  column,  close  to  Goll's.     (c)  The  crossed 


Entering  posterior 
root 


ssauer's  tract 


Fig.  71.  Diagram  representing  a  transverse  section  through  the  spinal  cord.  (Modi- 
fied from  Cunningham.)  A,  bundle  of  Helweg.  B,  descending  antero-lateral  tract. 
C,  septo-marginal  tract.     D,  comma.     E,  pre-pyramidal  tract. 

pyramidal  tract  (fasciculus  cerebro -spinalis  lateralis),  and  the  pre= 
pyramidal  tract  {rubrospinal  tract ;  von  Monakow's  tract)  lying  in  the 
lateral  column,  (d)  The  spino=o!ivary  column  (bundle  of  Helwig) 
lying  superficially  opposite  the  anterior  horn,  (e)  The  anterolateral 
descending  tract  (vestibulospinal  tract;  marginal  bundle  of  Loiven- 
thal),  marginally  in  front  of  the  antero-lateral  ascending  tract,  (f)  The 
direct  pyramidal  tract  (fasciculus  cerebro  spinalis  anterior),  along  the 
border  of  the  anterior  fissure.  The  fibers  in  the  pyramidal  tract  are  axons 
of  cell-bodies  in  the  cerebral  cortex.  The  cell-bodies  of  the  fibers  in  the 
other  descending  columns  lie  in  the  medulla,  pons,  cerebellum,  or  mid- 
brain, or  grav  columns  of  the  cord. 


82  PSYCHOBIOLOGY 

III.  Spinal    interconnecting.      The   basic    bundles,    anterior    and 

lateral  [fasciculus  anterior  proprius  and  fasciculus  lateralis  proprius), 
are  strands  of  fibers  of  cells  in  the  gray  matter  of  the  cord,  serving  to  con- 
nect the  different  segments  of  the  cord  with  one  another. 

THE  SPINAL  AND  CRANIAL  NERVES. 

The  spinal  nerves  issue  from  the  cord  in  pairs,  a  pair  for  each  articula- 
tion of  the  spinal  column.  The  nerve  on  each  side  has  two  roots,  that  is, 
it  is  made  up  out  of  two  sets  of  fibers,  one  set  efferent,  issuing  from  the 
anterior  horn  of  the  gray  matter  of  the  cord,  and  the  other  (afferent) 
entering  the  cord  on  the  posterior  side.  The  spinal  ganglion  lies  on  the 
posterior  root  and  the  two  roots  are  united  in  the  intraspinal  foramen  just 
beyond  the  ganglion,  forming  a  single  nerve. 

There  are  31  pairs  of  spinal  nerves,  which  are  named,  according  to  the 
position  of  their  origins  in  the  spinal  cord,  cervical  (8),  thoracic  (12), 
lumbar  (5),  sacral  (5),  and  coccygeal  (1).  These  nerves  are  num- 
bered in  each  group  downwardly ;  thus,  the  uppermost  spinal  nerve  is  the 
'  1st  cervical ',  the  next,  the  '  2nd  cervical ',  the  ninth,  the  '  1st  thoracic  ', 
and  so  on. 

The  nerves  issuing  above  the  1st  cervical  are  called  cranial  nerves, 
and  are  numbered  from  one  to  twelve.  When  a  nerve  is  referred  to  by 
number  with  no  region  assigned,  '  cranial '  is  always  meant.  Only  four  of 
the  cranial  nerves  are,  like  the  spinal,  '  mixed  nerves  '.  Of  the  other  eight, 
three  are  pure  afferent,  or  '  sensory ',  and  five  are  efferent,  or  '  motor  '. 

The  cranial  nerves,  in  order,  as  they  are  conventionally  numbered,  are 
given  in  the  following  list : 

I.  The  olfactory  nerve  (afferent).  This  is  not  a  'nerve'  in  the 
usual  sense,  but  a  number  of  nerves,  not  collected  in  a  bundle,  running 
from  the  olfactory  membrane  in  the  nose  to  the  olfactory  bulb. 

II.  The  optic  nerve  (afferent)  is  composed  principally  of  axons  from 
the  ganglion  cells  in  the  retina  of  the  eye.  These  pass  back  to  the  '  optic 
chiasm  ',  where  the  fibers  from  the  left  half  of  the  eye  continue  in  the  left 
optic  tract,  and  the  fibers  from  the  right  half,  in  the  right  optic  tract, 
back  to  the  external  geniculate  body,  the  pulvinar  of  the  thalamus,  and 
the  superior  corpora  quadrigemina.  From  the  first  two  of  these  primary 
visual  centers,  fibers  pass  to  the  visual  cortex.  From  the  third,  fibers 
go  to  the  oculo=motor  center,  which  controls  the  contraction  of  the  eye- 
muscles.  Fibers  also  pass  outward  to  the  eye  in  the  optic  nerve  from  the 
primary  centers.  Some  of  these  are  from  the  oculo-motor  center  (just 
mentioned),  but  there  are  also  fibers  from  the  other  two  centers. 

III.  The    oculo=motor    nerve     (afferent)     arises    from    an    extensive 


Nerves,  Spinal  Cord,  Brain  and  Other  Ganglia         83 

nucleus  lying  along  the  front  and  central  portion  of  the  brain  stem  just 
above  the  pons  (in  the  '  aqueduct  of  Sylvius'  and  the  'third  ventricle'), 
emerging  from  the  inner  margin  of  the  crusta.  The  cells  in  the  anterior 
part  of  this  nucleus  send  axons  to  the  intrinsic  muscles  of  the  eyes  (the 
ciliary  muscle  and  the  sphincter  pupillae) .  The  cells  of  the  remainder 
of  the  nucleus  send  axons  to  certain  extrinsic  muscles  of  the  eye,  viz., 
the  recti  (except  the  external  rectus),  the  inferior  oblique,  and  the  levator 
palpebrarum. 

Along  with  these  axons,  dendrites  from  cells  in  the  nuclei  are  sent  to 
the  same  muscles.  These  afferent  neurons,  whose  cell  bodies  are  located 
in  the  same  ganglia  with  the  bodies  of  efferent  neurons,  with  the  distribu- 
tion of  whose  axons  the  distribution  of  the  dendrites  is  coextensive,  are 
called  proprioceptive  neurons. 

IV.  The  trochlear  or  pathetic  nerve  (efferent  and  afferent)  arises 
from  a  nucleus  behind  that  of  the  third  nerve,  in  the  floor  of  the  aqueduct 
of  Sylvius  at  the  level  of  the  inferior  corpora  quadrigemina.  The  fibers 
emerge  through  the  valve  of  Vieussens  (the  thin  plate  which  forms  part 
of  the  anterior  wall  of  the  '  fourth  ventricle  '  in  front  of  the  cerebellum) . 
The  axons  forming  this  nerve  run  to  the  superior  oblique  muscle  of  the 
eye. 

V.  The  trigeminal  or  trifacial  nerve  (afferent  and  efferent)  corres- 
ponds to  a  spinal  nerve,  having  afferent  fibers  from  cells  in  the  '  Gasserian 
ganglion  ',  which  send  their  axons  into  the  pons,  and  efferent  fibers  arising 
from  nuclei  in  the  lateral  walls  of  the  fourth  ventricle  and  higher  in  the 
brain  stem.  The  afferent  axons  branch,  like  the  fibers  of  spinal  nerves, 
the  ascending  branches  going  to  a  nucleus  in  the  upper  part  of  the  pons, 
the  descending  branches  passing  forward  as  far  as  the  cervical  part  of  the 
spinal  cord. 

The  afferent  dendrites  run  to  the  face,  including  mouth,  eye-balls,  and 
nose.  The  efferent  axons  run  to  the  muscles  of  mastication,  and  the 
tensor  tympani  muscles  of  the  ears  and  the  tensor  palati. 

VI.  The  abducent  nerve  (afferent  and  efferent)  arises  from  a  nucleus 
of  cells  growing  in.  the  floor  of  the  upper  part  of  the  fourth  ventricle  on 
each  side  of  the  middle  line,  emerging  from  the  anterior  side  of  the  med- 
ulla at  the  lower  edge  of  the  pons.  The  axons  of  the  sixth  nerve  run  to 
the  external  rectus  muscles  of  the  eyes,  and  are  accompanied  by  proprio- 
ceptive dendrites  as  in  the  case  of  the  third  and  fourth  nerves.9 

9  The  nuclei  of  the  iiird,  fourth,  and  sixth  nerves  receive  collateral  fibers  ('com- 
missural ')  from  the  posterior  longitudinal  bundle,  man}'  of  which  are  axons  from 
cells  in  '  Deiter's  nucleus '.  By  means  of  these  commissural  fibers,  doubtless,  the 
actions   of  the   various  eye  muscles   are   coordinated. 


84  PSYCHOBIOLOGY 

VII.  The  facial  nerve  (afferent  and  efferent)  has  its  nucleus  for  ef- 
ferent fibers  in  the  medulla  behind  the  superior  olive,  from  which  the 
fibers  emerge  somewhat  above  this  point  on  the  sides  of  the  pons  and  run 
to  the  muscles  of  the  face,  scalp  and  external  ear.  The  afferent  dendrites 
originate  in  the  '  geniculate  ganglion ',  from  which  some  of  them  run  to 
certain  of  the  gustatory  organs  in  the  tongue,  and  some  probably  to  end- 
ings in  the  muscles  supplied  by  the  axon  of  this  nerve  (proprioceptive, 
therefore).  These  fibers  leave  the  main  nerve  through  the  'nerve  of 
Wrisberg  '.  The  afferent  axons  have  ascending  and  descending  branches 
like  those  of  spinal  nerves. 

VIII.  The  auditory  nerve  (afferent)  joins  the  medulla  close  to  the 
outside  of  the  seventh  nerve.  The  nerve  is  composed  of  axons  of  bipolar 
cells  in  the  cochlea  (on  the  cochiear  branch)  and  in  the  vestibule  of  the 
ear  (on  the  vestibular  branch).  On  entering  the  medulla,  the  axons 
from  the  cochlea  division  branch,  the  ascending  branches  terminating  in 
the  '  ventral '  or  '  accessory '  nucleus,  and  the  descending  branches  in  the 
'dorsal'  or  'principal'  nucleus  (acoustic  tubercle).  From  the  nuclei 
new  relays  of  axons  cross  upwards  to  the  opposite  sides  of  the  medulla 
and  run  upwards  in  the  lateral  fillets,  some  being  relayed  in  the  superior 
olivary  nucleus,  and  all  terminating  in  the  posterior  corpora  quadri= 
gemina  (and  internal  geniculate  bodies?). 

The  fibers  of  the  vestibular  division  of  the  eighth  nerve  also  send  their 
branches  into  the  '  ventral '  and  '  dorsal '  nuclei',  the  fibers,  or  at  least 
many  of  them,  running  through  the  nuclei  into  the  cerebellum,  where  they 
connect  with  cells  in  the  '  roof  '  nucleus. 

IX.  The  glossopharyngeal  nerve;  X.  The  pneumogastric  or  vagus 
nerve;  and  XI.  The  spinal  accessory  nerve,  have  their  roots  in  a  series 
of  bundles  in  the  sides  of  the  medulla.  The  efferent  axons  of  these  nerves 
come  from  cells  in  the  dorsal  nucleus  of  the  vagus  and  accessory  nerves 
lying  externally  to  the  nucleus  of  the  twelfth  nerve  (hypoglossal  nucleus) 
(v.  infr.)    and  from  cells  in  the  nucleus  ambiguus  which  lies  deeper  in 

the  medulla. 

The  axons  of  the  ninth  nerve  run  to  the  muscles  of  the  pharynx  and  the 
base  of  the  tongue,  and  to  the  parotid  gland.  The  afferent  dendrites  origi- 
nate in  cells  of  the  ganglion  petrosum  and  ganglion  superius,  and  run  to 
the  mucous  membrane  of  the  tongue,  mouth  and  pharynx. 

The  efferent  axons  of  the  tenth  nerve  run  to  the  levator  palati  and  the 
three  constrictors  of  the  pharynx,  the  muscles  of  the  larynx,  the  muscular 
Avails  of  the  esophagus,  stomach  and  small  intestine,  certain  smooth 
muscles  in  the  walls  of  the  bronchi  and  bronchioles,  to  the  glands  of  the 
-stomach  and  possibly  to  the  pancreas.     In  addition,  inhibitory  fibers  run 


Nerves,  Spinal  Cord,  Brain  and  Other  Ganglia         85 

to  the  heart.     The  afferent  axons  of  the  spinal  accessory  (eleventh)  nerve 
supply  the  sterno-mastoid  and  trapezius  muscles. 

XII.  The  hypoglossal  nerve  (efferent)  arises  from  cells  in  the  floor 
of  the  fourth  ventricle  at  its  lower  end,  close  to  the  middle  line,  and  issues 
from'  the  front  side  of  the  medulla  between  the  anterior  pyramid  and  the 
'  olivary  body  '.  The  axons  run  to  the  tongue  (anterior  portions)  and  ex- 
trinsic muscles  of  the  larynx,  and  those  moving  the  '  hyoid  '  bone. 

REFERENCES  ON  GROSS  RELATIONS  OF  NERVE,  CORD,  BRAIN,  AND  GANGLIA. 

Starling,  Physiology,  Ch.  VII,  §§  VI,  X,  XI,  XV  and  XVIII. 

Cunningham,  Anatomy,  §  The  Nervous  System. 

Villiger,  The  Brain  and  Spinal  Cord   (Piersol's  Translation). 

Schafer,    Quain's   Anatomy,   Vol.    Ill,    Pt.    I    (General    Neurology    and    the    Central 

Nervous  System). 
Howell,  Physiology,  Chs.  VIII-XI. 
Lewis  and  Stohr,  Histology,  §  III,  Sub-§  The  Central  Nervous  System. 


CHAPTER  VII. 

THE  VISCERAL    OR   SPLANCHNIC   DIVISION   OF    THE   NERVOUS   SYSTEM. 

Up  to  this  point  our  discussion  of  nervous  structures  has  been  chiefly  of 
those  neurons  whose  bodies  are  located  in  the  brain  and  cord,  or  which 
connect  the  brain  or  cord  with  striped  muscle  or  the  organs  of  external 
sense.  These  neurons,  taken  together,  are  properly  said  to  constitute  the 
somatic  division  of  the  nervous  system.  Sometimes  this  somatic  division 
is  designated  as  the  cerebro=spinaI  system;  a  designation  which  is  per- 
niciously misleading,  since  the  cerebro-spinal  system  (if  the  term  is  to  be 
used  at  all ) ,  includes  more  than  the  somatic  division ;  includes,  in  fact, 
all  nervous  tissue  except  the  cells  of  the  '  local '  plexuses,  described  below. 

Those  neurons  which  supply  afferent  and  efferent  connections  between 
the  brain  and  cord  and  the  viscera — the  smooth-muscle  tissues  of  the  blood 
vessels,  skin,  alimentary  canal,  etc.,  and  the  glandular  tissues — are  collec- 
tively designated  as  the  visceral,  splanchnic  or  autonomic  division  of 
the  nervous  system.  Frequently,  the  word  '  division  '  is  dropped  out  and 
the  expressions  'splanchnic  system'  (or  visceral  or  autonomic,  etc.)  and 
'  somatic  system  '  are  used.  It  is  to  be  remembered,  however,  that  the 
somatic  and  the  greater  part  of  the  splanchnic  divisions  are  really  parts 
of  the  cerebro-spinal  system.  There  is  some  confusion  in  the  use  of  terms 
referring  to  the  splanchnic  system,  and  even  in  the  classification  of  the 
parts  of  the  total  system.  The  terminology  and  classification  herein  are  in 
accordance  with  Starling,  who  is  a  good  authority  to  follow  in  this  matter. 

The  distinguishing  peculiarity  of  the  visceral  system  of  nerves,  from 
the  morphological  point  of  view,  is  the  fact  that  the  connection  between 
the  brain-stem  or  spinal  cord  and  the  structures  supplied  by  the  nerve- 
fibers  is  a  two -neuron  connection.  This  is  true  at  least  of  the  efferent 
neurons.  As  regards  the  afferent,  information  seems  to  be  lacking.  Com- 
petent authorities  agree  that  the  afferent  visceral  neurons  of  the  spinal 
roots  have  their  cell  bodies  in  the  spinal  ganglia,  as  do  the  peripheral 
afferent  somatic  neurons,  but  do  not  decide  whether  these  neurons  extend 
to  the  visceral  periphery  or  connect  synaptically  in  the  ganglia  with  a 
second  set  of  neurons,  as  do  the  efferent  visceral  neurons.  The  efferent 
neuron  which  has  one  termination  (and  usually  its  cell  body)  in  the  cord 
(or  in  the  brain  stem)  has  its  other  termination  in  a  ganglion  at  a  greater 


The  Visceral  or  Splanchnic  Division 


87 


or  less  distance  from  the  cord  or  brain,  and  the  fiber  (from  cord  or  brain- 
stem to  ganglion)   is  called  a  pre=gangIionic  fiber.     From  the  ganglion 


Fig.  72.  The  distribution  and  connections  of  the  sympathetic  and  vagus  nerves  on 
the  right  side.  (Quain's  Anatomy,  Vol.  Ill,  Pt.  II,  after  Hirschfeld  and  Laveille.) 
The  sympathetic  chain  of  ganglia  is  shown  from  the  inferior  cervical  ganglion  down 
to  the  first  lumbar  ganglion  (58).  Ganglia:  4,  ciliary;  5,  spheno-palatine  ;  6,  otic;  7, 
submaxillary;  21,  33,  38,  superior,  middle  and  inferior  cervical;  48,  semi-lunar. 
Glands:  a,  lachrymal;  d,  thyroid.  A,  Heart,  g,  Stomach.  28,  Vagus  nerve.  47, 
Great  splanchnic  nerve.     Plexuses:  42,  cardiac;  50,  solar;  53,  gastric. 

the  connection  with  the  smooth  muscle  or  gland  is  completed  by  an  axon 
or  dendrite  of  a  second  neuron,  called  a  post=ganglionic  fiber. 


88 


PSYCHOBIOLOGY 


The  visceral  division  comprises  several  subdivisions,  which  may  be 
grouped  under  these  heads.  1.  The  sympathetic  division  (or  system), 
so  called  because  it  was  formerly  believed  to  be  capable  of  reflexes  inde- 
pendent of  the  cerebro-spinal  mechanism.  2.  The  vagal,  cranial,  and 
sacral  nerves  and  ganglia.  3.  The  '  local '  systems  of  the  alimentary 
canal. 

1.  The  sympathetic  division  comprises  the  sympathetic  chains  of 
ganglia,  one  chain  lying  on  each  side  of  the  vertebral  column;  and  the 
collateral  ganglia  in  special  relation  to  the  abdominal  viscera.  There 
i?  (on  each  side)  one  sympathetic  ganglion  for  each  of  the  spinal  nerve 
roots  from  the  fifth  thoracic  to  the  third  sacral;  there  is  one  ganglion 
(the  '  stellate  '  ganglion)  connected  with  the  first  four  thoracic  roots,  and 
two  ganglia  (the  'inferior  and  superior  cervical')  associated  with  the 
eight  cervical  roots. 


Fig.  73.  The  connection  of  a  spinal  nerve  with  a  ganglion  of  the  sympathetic  chain. 
(Quain's  Anatomy.)  The  two  rami  connecting  the  sympathetic  ganglion  with  the 
spinal  nerve  are  shown,  and  also  the  two  roots  of  the  recurrent  nerve  which  supplies 
the  tissues  lying  within  the  spinal  canal  surrounding  the  spinal  cord. 

There  are  three  collateral  ganglia  lying  near  the  points  at  which  the 
large  arteries  originate  from  the  aorta;  these  are  the  superior  mesenteric 
ganglion,  the  inferior  mesenteric  ganglion,  and  the  semilunar  10  or  solar 
ganglion. 

The  pre-ganglionic  fibers  of  the  sympathetic  division  leave  the  spinal 
nerves  over  the  white  rami  communicantes  of  each  nerve,  and  some  of 
the  post-ganglionic  fibers  are  again  given  back  over  the  gray  rami  to  the 
spinal  nerves  for  distribution  to  various  parts  of  the  body.11  The  white 
rami  are  largely  composed  of  medullated  fibers,  and  the  gray  rami  are 


10  Not  to  be  confused  with  the   Gasserian  ganglion   (cranial),  which  unfortunately 
is  also  called  the  semilunar  ganglion. 

11  The  majority  of  the  post-ganglionic  fibers  of  the  '  sympathetic  '   division  do  not 
return  to  the  spinal  nerves,  but  emerge  through  the  visceral  nerves. 


The  Visceral  or  Splanchnic  Division 


89 


largely  composed  of  non-medullated  fibers;  hence  the  difference  in  color. 
The  pre-ganglionic  fibers  do  not  necessarily  terminate  in  the  ganglia  near- 
est to  the  white  rami  over  which  they  run.  Some  fibers  pass  up  or  down 
the  chain  to  other  lateral  ganglia,  and  others  run  through  to  the  collateral 
ganglia.12    The  post-ganglionic  fibers  given  back  to  a  spinal  nerve  through 


Fig.  74.  Diagram  of  the  motor  connections  of  the  sympathetic  chain.  (Quain's 
Anatomy,  after  Van  Gehuchten.) 

its  gray  ramus  are  connected  with  the  pre-ganglionic  fibers  of  a  number 
of  white  rami. 

The  distribution  of  the  post-ganglionic  sympathetic  fibers  and  the 
spinal  origin  of  the  pre-ganglionic  fibers  with  which  they  are  connected 
may  be  indicated  briefly. 

The  first  five  thoracic  nerve-roots,  chiefly  the  second  and  third,  supply 
the  head  and  neck  by  way  of  the  '  superior  cervical '  ganglion,  and  the 

12  Some  fibers  run  through  both  lateral  and  collateral  ganglia,  terminating  in 
peripheral  ganglia   (or  terminal  ganglia)    in  close  relation  to  the   organs   supplied. 


90  PSYCHOBIOLOGY 

heart  and  lungs  through  the  stellate  ganglion.  The  lower  thoracic  and 
upper  three  or  four  lumbar  spinal  roots  supply  the  abdominal  viscera 
(stomach,  small  intestine,  kidney,  spleen),  by  way  principally  of  the  '  col- 
lateral '  ganglia ;  and  the  colon,  bladder,  and  genital  organs  by  way  of  the 
'  pelvic  '  ganglia.  The  arm  is  innervated  from  the  thoracic  roots  from  the 
fourth  to  the  tenth,  through  the  'stellate'  ganglion;  and  the  leg  is  sup- 
plied from  the  roots  from  the  thoracic  dorsal  to  the  third  lumbar,  through 
lateral  ganglia  of  the  lumbar  and  sacral  regions. 

The  functions  of  the  '  sympathetic  '  nerves  are,  in  brief :  to  cause  con- 
traction and  relaxation  of  the  muscular  coats  of  the  blood  vessels  (which 
functions  are  called  vasoconstrictor  and  vaso=diIator  respectively)  ; 
to  cause  contraction  and  relaxation  of  the  smooth  muscle  of  various  other 
viscera  (motor  and  inhibitory  functions)  ;  to  stimulate  secretion  of  sali- 
vary and  sweat  gland ;  and  to  accelerate  the  heart  beat. 

2.  Visceral  neurons  of  the  cranial,  vagus,  and  sacral  divisions. 

The  third,  seventh,  ninth,  tenth  and  eleventh  cranial  nerves  contain 
visceral  fibers,  as  well  as  somatic  fibers.  The  visceral  fibers  in  the  third 
nerve  are  axons  which  run  to  the  '  ciliary  '  ganglion  in  the  orbit  (eye 
socket)  from  which  the  impulses  are  relayed  to  the  ciliary  muscle  (the 
muscle  of  accommodation)  and  the  sphincter  pupillae  (muscle  of  the  iris). 
The  visceral  fibers  in  the  facial  (seventh)  nerve  are  efferent,  but  are  den- 
drites of  cell  bodies  lying  in  several  cranial  ganglia  ('submaxillary', 
'  spheno-palatine  '  ganglia,  etc.),  differing  thus  from  the  typical  arrange- 
ment of  the  efferent  neurons  conducting  from  the  cord  and  brain.  The 
fibres  relaying  from  these  ganglia  terminate  in  the  sublingual  and  sub- 
maxillary glands  (salivary),  the  blood  vessels  of  the  tongue  and  the 
glands  of  various  parts  of  the  mucous  membrane  of  the  nose  and  mouth 
cavities. 

The  visceral  fibers  in  the  glossopharyngeal  (ninth)  nerve  are  axons  and 
dendrites  of  cell-bodies  in  the  medulla,  and  run  to  the  otic  ganglion, 
whence  the  efferent  fibers  are  relayed  by  post-ganglionic  axons  to  the 
parotid  gland  (salivary).  Possibly  there  are  ninth-nerve  fibers  running 
to  blood  vessels  in  the  back  part  of  the  tongue.  The  tenth  nerve,  with 
some  of  the  fibers  derived  from  the  roots  of  the  eleventh,  together  form 
the  vagus,  or  pneumogastric  nerve,  which,  like  the  ninth  nerve,  is  en- 
tirely visceral,  and  both  afferent  and  efferent.  The  afferent  fibers  are 
dendrites  derived  from  the  '  jugular  '  ganglion,  and  the  ganglion  '  trunci 
vagi'  (vagus  trunk  ganglion)  ;  the  efferent  fibers  are  (like  those  of  the 
seventh  nerve)  dendrites  of  cell-bodies  in  the  ganglia  located  in  the  vis- 
cera the  nerve  supplies.  The  efferent  distribution  of  the  vagus  is  to  the 
smooth   muscles    of   the    gullet,    stomach,    small    intestine,    and   bronchial 


The  Visceral  or  Splanchnic  Division  91 

tubes;  to  the  gastric  glands  of  the  stomach,  and  possibly  of  the  pancreas; 
and  to  the  heart.  The  effect  of  vagus  currents  on  the  heart  is  solely  in- 
hibitory, i.  e.j  decreasing  the  activity  of  the  cardiac  muscle.  The  distribu- 
tion of  the  afferent  fibers  is  not  so  clearly  known. 

The  pelvic  or  sacral  visceral  connections  of  the  central  nervous  system 
all  run  in  the  pelvic  visceral  nerve  (nervus  erigens) ,  and  are  axons  of 
spinal  cell-bodies.  These  fibers  terminate  in  the  pelvic  ganglia,  lying  in 
the  neighborhood  of  the  bladder,  from  which  the  further  connections  are 
with  the  muscles  of  the  bladder,  colon,  rectum  and  sexual  organs  (and  the 
blood  vessels  therein). 

3.  Local  nervous  systems. 

The  neurons  described  under  1  and  2  belong  definitely  to  the  central 
nervous  system,  as  will  be  explained  below.  There  are,  however,  certain 
groups  of  neurons  which  have  not  such  direct  connection  with  the  cerebro- 
spinal apparatus.  These  are  the  plexuses  of  Auerbach  and  of  Meissner, 
located  in  the  walls  of  the  alimentary  canal  from  esophagus  to  rectum, 
and  serving  as  '  centers  '  for  this  series  of  organs.  They  form,  in  other 
words,  an  independent  local  system,  with  afferent  and  efferent  fibers  hav- 
ing (probably)  no  communication  with  the  general  nerve  system.  Stimu- 
lation of  the  sensory  terminals  of  neurons  in  this  local  system  may  there- 
fore be  transmitted  by  a  relatively  short  circuit  to  the  smooth  muscle  of 
the  coats  of  the  gullet,  stomach,  and  intestines. 

GANGLIA   OF    THE   VISCERAL    SYSTEM    AND    THEIR    FUNCTIONS. 

Ignoring  now  the  local  visceral  systems  just  described,  we  find  that  the 
visceral  system  of  neurons  involves,  in  addition  to  the  structures  in  the 
spinal  cord  and  the  spinal  ganglia,  two  sets  of  ganglia.  1.  The  chain  of 
lateral  ganglia,  and  the  collateral  ganglia,  of  the  '  sympathetic  '  division ; 
and  2,  certain  peripheral  ganglia  (/.  e.}  ganglia  located  at  a  greater  or  less 
distance  from  the  cerebro-spinal  apparatus)  of  the  cranial,  sacral,  and 
vagus  visceral  nerves.  Among  peripheral  ganglia,  for  instance,  are  the 
otic,  orbital,  submaxillary,  and  pelvic  ganglia  earlier  mentioned.  These 
ganglia  are  sometimes  called  '  nerve  centers  ',  and  properly  come  under 
■one  of  the  several  meanings  of  that  highly  confusing  term.  But  they  are 
not  (and  this  is  important),  structures  in  which  afferent  currents  are  con- 
verted into  efferent  currents.  No  reflex,  in  other  words,  can  take  place 
through  these  ganglia  alone ;  the  afferent  current  must  go  on  into  the  cord, 
even  if  it  does  not  go  up  to  the  brain,  before  it  can  be  redirected  outward 
to  any  of  the  effectors.  The  ganglia  of  the  visceral  nerves  have  merely  a 
distributory  function.  A  current  passing  out  from  the  spinal  cord  in  an 
axon  which  leaves  the  spinal  nerve  over  the  white  ramus,  is  distributed. 


92  PSYCHOBIOLOGY 

in  one  of  the  lateral  or  collateral  ganglia,  to  a  number  of  cells  therein,, 
and  its  distribution  in  the  tissues  reached  by  the  axons  of  these  cells 
thereby  increased.  The  current  transmitted  through  a  single  white  ramus 
is  in  many  cases  returned  through  the  gray  rami  of  many  spinal  nerves, 
and  passes  along  fibers  in  the  nerves  to  many  portions  of  the  body.  It  is 
not  impossible  that,  conversely,  afferent  currents  from  a  relatively  large 
area  may  be  collected  in  one  of  these  ganglia  by  the  dendritic  branches  of 
a  single  spinal  ganglion  cell.  On  this  point,  however,  definite  information 
is  not  at  hand. 

THE  STIMULATION  OF  AFFERENT  VISCERAL   NEURONS. 

The  excitability  of  the  afferent  neuron  terminations  in  the  walls  of  the- 
alimentary  canal,  especially  the  terminations  of  the  visceral  afferent  neurons, 
belonging  to  the  central  nervous  system,  has  long  been  a  subject  for  study 
and  speculation.  For  a  long  time  it  was  believed  that  these  terminations, 
at  least  those  below  the  gullet,  although  afferent  were  not  sensory,  i.  e.y 
that  no  consciousness  could  be  produced  or  mediated  through  their 
activity.  It  was  shown  that  no  specific  sensations  were  experienced  by  a 
patient  when  his  intestines  were  handled,  pressed  upon,  cut,  electrically 
stimulated,  or  even  torn  or  burned.  The  peritoneal  membranes  covering 
the  intestine,  and  lining  the  abdominal  cavity,  were  found  sensitive  in  one 
particular,  i.  e.,  '  pain  '  was  produced  by  strong  stimulation ;  but  no  con- 
sciousness seemed  to  follow  any  operation  on  the  intestines  themselves. 

The  pain  of  colic  and  other  less  acute  discomfort  localized  in  the  ab- 
dominal cavity  were  concluded  to  be  due  to  peritoneal  irritation. 

Later  and  more  adequate  experiments  have  shown,  however,  that  certain 
afferent  currents  from  the  intestines  do  produce  consciousness,  but  that 
these  afferent  currents  are  initiated  only  when  the  nerve  terminals  are 
stimulated  adequately,  that  is,  in  this  case,  when  the  intestinal  muscular 
fibers  are  contracted  or  stretched.  Thus  we  derive  the  experiences  of 
pain  (as  of  colic).  Hunger  is  due  to  contraction  of  the  stomach;  full- 
ness to  stretching  of  the  stomach  walls,  and  probably  feelings  of  '  faint- 
ness  '  and  satisfaction,  and  others  not  readily  named,  are  due  to  other 
variations  in  the  stimulation  of  these  organs. 

Terminals  of  the  local  systems  (of  Auerbach  and  Meissner)  are  excitable 
through  pressure  of  the  contents  of  the  canal  on  the  lining  membrane,  and 
through  the  chemical  substances  contained  in  food,  and  in  secretions  of 
other  regions  of  the  canal.  In  this  way  the  internal  control  of  the  diges- 
tive process  is  maintained.  Whether  afferent  terminals  belonging  to  the 
central  system  may  be  chemically  stimulated  is  an  open  question.  As  to 
the  stimulation  of  afferent  visceral  terminals  in  connection  with  tissues  in 


The  Visceral  or  Splanchnic  Division  93 

the  skin,  blood  vessels  and  glands,  and  the  specific  effect  thereof,  we  have 
yet  all  to  .learn. 

REFERRED   PAIN. 

In  certain  pathological  conditions  of  the  visceral  organs,  the  pain  which 
is  felt  is  falsely  localized  in  the  skin.  This  association  of  skin  areas  with 
visceral  regions  is  definite  and  specific,  and  by  the  exact  area  of  the  skin 
which  seems  sore  (although  the  skin  is  really  normal),  it  is  possible  to 
diagnose  the  exact  visceral  region  affected.  The  linkage  of  skin  and  vis- 
cera is  doubtless  through  associative  neurons  located  in  the  spinal  ganglia. 
Such  neurons  have  been  discovered,  and  probably  join  cell-bodies  of  vis- 
ceral neurons  with  cell-bodies  of  somatic  neurons.  In  this  way  it  would 
be  possible  for  currents  entering  through  the  visceral  channels  to  be 
switched  off  to  the  somatic  neurons  and  continue  upwards  over  that  route, 
although  the  transfer  does  not  occur  unless  there  is  pathological  irritation. 

REFERENCES  ON  THE  VISCERAL  DIVISION  OF  THE  NERVOUS  SYSTEM. 

Langley,  The  Sympathetic  and  Other  Related  Systems  of  Nerves.  Schafer's  Text- 
Book  of  Physiology,  Vol.  II. 

Starling,  Physiology,  Chapter  VII,  §  XVIII. 

Howell,  Physiology,  Chapter  XII. 

Herz,  The  Sensibility  of  the  Alimentary  Canal.     London:  Frowde,   1911. 

Cr.nnon  and  Washburn,  An  Explanation  of  Hunger,  American  Journal  of  Physiology, 
1912,  Vol.  XXIX,  pp.  441-454. 

See  also  articles  by  Cannon  and  his  pupils  in  the  1913  and  1914  number  of  the 
American  Journal  of  Physiology. 


CHAPTER  VIII. 

GLANDS. 

Glands  are  organs  of  secretion  or  of  excretion.  In  structure  they  in- 
volve all  five  fundamental  bodily  tissues — epithelial,  connective  and  vas- 
cular in  all  cases,  nervous  tissue  in  nearly  all,  and  muscular  tissue  in  the 
larger  glands — but  the  secreting  or  excreting  agents  in  the  glands  are  cells 
of  epithelial  origin. 

Secretion  is  the  production  from  the  bodily  fluids — or,  in  some  cases, 
the  mere  separation  from  the  bodily  fluid — of  substances  which  are  directly 
useful  to  cells  other  than  those  secreting  them,  or  which  are  useful  to  the 
organism  as  a  whole :  e.  g.,  saliva,  produced  by  the  salivary  glands,  is  nec- 
essary for  the  digestion  of  starch ;  and  sweat,  produced  by  glands  in  the 
skin,  helps  to  regulate  the  skin  temperature.  Excretion  is  the  separation 
or  the  elimination  from  the  bodily  fluids  of  waste  products  of  cells  other 
than  those  excreting  them:  e.  g.,  urine,  excreted  by  the  kidneys.  Some- 
times a  gland  combines  both  functions,  as  in  the  case  of  the  liver,  the  se- 
cretion of  which  (bile)  both  contains  waste  products  and  is  also  an  im- 
portant agent  in  intestinal  digestion.  The  processes  of  secretion  and 
excretion  are,  in  a  sense,  common  to  all  cells.  All  cells  take  up  from  the 
blood  and  lymph  substances  required  for  their  own  nutritive  processes  and 
give  off  waste  products.  But  technically  the  terms  '  excretion  '  and  '  secre- 
tion '  apply  only  to  the  processes  as  described  above,  viz.,  in  which  certain 
specialized  cells  are  carrying  on  the  functions  for  the  direct  benefit  of  other 
cells. 

There  are  many  secreting  cells — gland  cells — which  are  not  located  in 
glands,  but  are  scattered  in  epithelial  membranes,  especially  in  the  mucous 
membrane.  Such  are  the  '  goblet  cells  ',  earlier  described.  Goblet  cells, 
which,  during  their  period  of  activity,  form  a  mass  of  secretion  within 
themselves  and  then  liberate  it  at  the  end  of  the  period  of  activity,  repre- 
sent the  middle  type  of  gland-cell.  At  one  extreme  are  cells  which  prob- 
ably produce  and  liberate  their  secretions  continuously  during  the  active 
period :  such  are  the  cells  of  the  parotid  gland.  At  the  other  extreme  are 
cells  which,  having  become  filled  with  products  during  the  active  period, 
are  themselves  broken  up,  and  mingle  with  their  secretions  in  the  process 
of  discharging  them ;  such  are  the  lacteal  cells  in  the  mammary  glands, 
and  the  cells  of  the  sebaceous  glands  in  the  skin. 


Glands  95 

Glands  are  classified,  according  to  their  morphology,  first,  and  in  gen- 
eral, as  duct=glands  and  ductless  glands.  The  duct-glands  are  again 
classified  as  simple  and  compound,  and  also  as  tubular,  saccular  (or 
alveolar),  and  solid. 

Ductless  glands  discharge  their  products  directly  into  the  blood  stream, 
or  into  the  lymph,  so  that  the  veins  and  lymphatic  vessels  draining  these 
glands  may  be  considered  as  also  their  ducts.  The  products  of  these  glands 
are  frequently  designated  as  internal  secretions,  or  more  technically, 
hormones.  Ductless  glands  are  of  course  secretory  only,  and  the  secre- 
tion is  clearly  a  process  of  manufacture,  i.  e.,  the  blood  leaving  these 
organs  contains  substances  not  contained  in  the  entering  blood.  The  prac- 
tical use  of  these  substances  (hormones)  is  to  excite  or  sensitize  cells  to 
which  the  blood  stream  carries  them. 

Duct-glands  produce,  or  separate  from  the  blood  or  lymph,  substances 
which  are  needed  for  specific  purposes  outside  of  the  body  tissues,  or  of 
which  the  body  needs  to  get  rid.  The  ducts  through  which  these  glandular 
products  are  delivered  to  the  proper  points  are  therefore  essentially  separ- 
ate from  the  other  connections  of  the  gland.  The  products  of  the  duct- 
glands  are  commonly  designated  as  external  secretions. 

DUCT-GLANDS. 

The  duct-glands  are  sometimes  referred  to  as  true  glands;  sometimes 
they  are  designated  simply  as  glands;  the  intention  in  this  case  being  that 
the  term  '  gland  '  shall  always  signify  the  duct-gland  when  not  qualified 
by  the  adjective  '  ductless  '.  These  usages  are  due  to  the  fact  that  the 
duct-glands  were  known  and  studied  before  it  was  known  that  the  ductless 
glands  are  secretory  organs  also,  and  the  term  '  gland  '  has  accordingly 
seemed  to  belong  to  the  former  alone.  This  terminology  is  needlessly  con- 
servative ;  the  ductless  glands  are  as  truly  secreting  organs  as  the  duct- 
glands  and  have  as  good  a  claim  to  the  designation.  It  is  unfortunate 
that  we  have  not  terms  more  easily  distinguished  than  '  duct '  and  '  duct- 
less ' :  cannidated  glands  is  a  perfectly  logical  term  for  duct-glands,  but 
it  is  not  in  use. 

.  The  simplest  duct-gland  is  a  pit  or  pocket  in  an  epithelial  surface,  lined 
with  secreting  cells.13  This  pit  may  be  tubular  in  shape,  whether  straight 
or  coiled,  or  may  be  saccular  (alveolar:  acinous),  i.  e.,  pouch-like,  with 
its  orifice  smaller  than  its  internal  cavity.  The  sweat  glands  in  the  skin, 
and  many  of  the  gastric  glands  in  the  stomach,   are  tubular :   the  only 

13  Man)'  of  the  simple  glands,  and  the  small  compound  glands,  although  their  duct- 
orifices  open  on  epithelial  surfaces,  lie  mainly  in  the  connective-tissue  layers  below 
the   epithelium. 


96  PSYCHOBIOLOGY 

simple  saccular  glands  found  in  the  human  body  are  a  few  of  the  sebaceous 
glands  of  the  skin.  In  compound  glands  the  lumen,  or  inner  part  of  the 
canal  of  the  glands,  is  branched ;  in  many  glands  the  lumen  branches  re- 
peatedly, so  that  the  canal  structure  is  very  complex.  Compound  glands 
may  be  tubular,  saccular,  or  sacculo=tubular  (acino-tubular ;  tubo-alveo- 
lar) .  The  compound  saccular  glands  are  often  designated  as  racemose 
glands.  The  kidneys  and  the  majority  of  the  gastric  glands  are  compound 
tubular.  The  salivary  glands  14  and  most  of  the  sebaceous  glands  of  the 
skin,  the  Meibomian  (or  tarsal)  glands  in  the  edges  of  the  eye-lid,  and 
the  mucous  glands  in  the  oral,  nasal  and  respiratory  passages  are  compound 
saccular. 

The  pancreas,  and  Brunner's  glands  in  the  small  intestine,  are  types 
of  compound  sacculo-tubular  glands.  The  acini  (sacs)  in  these  glands  are 
long  and  narrow  like  the  lumen-branches  of  the  compound  tubular  glands, 
but  are  distinguishably  larger  in  diameter  than  the  ducts  into  which  they 
discharge. 

The  liver  is  a  duct-gland,  but  it  does  not  belong  to  either  the  saccular 
or  tubular  classes.  In  glands  of  these  classes  there  is  a  cavity  (tube  or 
saccule),  or  a  number  of  cavities,  connected  with  the  same  duct,  and  the 
secreting  cells  line  these  cavities.  In  the  liver  the  secreting  cells  are  ar- 
ranged in  solid  masses  (no  cavities)  and  fine  branches  of  the  bile  duct  run 
everywhere  between  them.  It  is  therefore  classed  as  a  solid  gland  (Cun- 
ningham). 

The  compound  duct-glands  are  in  most  cases  surrounded  by  capsules 
of  connective  tissue  and  from  this  connective  tissue,  if  the  gland  is  very 
complex,  septa  extend  into  the  gland,  dividing  it  into  lobes  and  lobules. 
Each  lobe  or  lobule  contains  saccules  or  tubules  opening  into  a  common 
branch  of  the  duct.  The  secreting  cells  in  the  case  of  the  complex  glands 
are  confined  to  the  saccules  or  tubules,  the  cells  of  the  epithelial  lining  of 
the  duct  proper  being  non-secreting.  In  the  simple  glands  the  entire  lining 
of  the  cavity  may  be  secretory,  but  usually  in  these  also  the  secreting  cells 
are  confined  to  the  deeper  part  or  fundus,  the  more  superficial  portion 
being  merely  a  duct. 

The  glands  are  all  supplied  with  blood  vessels  and  lymph  vessels.  Most 
glands  are  supplied  with  nerves,  in  some  cases  from  both  the  autonomic 
and  the  direct  cerebro-spinal  systems. 

Some  of  the  gland  nerve  fibres  run  to  the  muscular  cells  of  the  blood 
vessels,  some  to  the  muscles  of  the  ducts,  and  some  to  the  secreting  cells 

14  According  to  Cunningham.  Howell  describes  them  as  tubular ;  Pierson  as  tubo- 
alveolar. 


Glands  97 

themselves.  The  activity  of  a  gland  can  be  altered  by  nerve  currents 
affecting  the  cells  directly  and  by  changes  in  the  volume  of  the  blood  sup- 
ply produced  by  contraction  or  enlargement  of  the  blood  vessels  as  well  as 
by  the  influence  of  substances  (such  as  C02  or  the  secretions  of  the  ductless 
glands)  brought  to  the  gland  cells  in  the  blood. 

THE  GENERAL  STRUCTURE  OF  THE  ALIMENTARY  CANAL. 

The  consecutive  gross  divisions  of  the  alimentary  canal  are  the  mouth 
cavity,  the  pharynx  or  throat,  the  esophagus  or  gullet,  the  stomach, 
the  small  intestine,  and  the  large  intestine.  The  stomach  connects 
with  the  gullet  through  the  esophageal  orifice  and  with  the  small  intes- 
tine though  the  pylorus.  The  small  intestine  is  divided  into  duodenum, 
jejunum  and  ileum,  the  first  being  the  upper  ten  or  eleven  inches  of  the  in- 
testine and  distinguished  from  the  remainder  both  structurally  and  func- 
tionally.   The  large  intestine  is  divided  into  ccecum,  colon,  and  rectum. 

The  entire  alimentary  canal  is  lined  with  mucous  membrane,  con- 
sisting of  a  surface  layer  of  stratified  epithelium  resting  on  a  layer  of  con- 
nective tissue  called  the  stroma  or  tunica  propria,  with  sometimes  a  base- 
ment membrane  separating  the  two.  Beneath  the  mucous  membrane  are 
muscular  and  connective-tissue  structures  which,  in  the  gullet,  stomach 
and  intestines,  take  on  the  form  of  definite  coats. 

The  four  coats  of  the  gullet,  stomach  and  intestines  are  therefore: 

1.  The  mucous  membrane.  The  lowest  stratum  of  the  stroma  (in  the 
organs  mentioned :  not  in  the  mouth  and  pharynx)  is  a  sheet  of  smooth 
muscle  fibres. 

2.  The  submucosa.  This  is  a  loosely  attached  layer  of  areolar  con- 
nective tissue. 

3.  The  muscular  coat.  In  the  upper  part  of  the  gullet  this  is  composed 
of  striated  muscle;  in  the  middle  portion,  of  both  smooth  and  striated 
fibers;  in  the  lower  portion  of  the  gullet  and  throughout  the  stomach, 
small  intestines,  caecum  and  colon,  there  are  smooth  fibers  only.  In  the 
gullet  and  intestines  the  muscle  fibers  are  arranged  in  two  layers,  an  inner 
circidar  layer  and  an  outer  longitudinal  layer.  '  In  the  stomach  there  are 
three  layers. 

4.  Surrounding  the  muscular  coat  of  the  upper  part  of  the  gullet  is  a 
coat  of  areolar  connective  tissue  loosely  joining  it  to  the  adjacent  struc- 
tures. The  lower  part  of  the  gullet,  stomach  and  intestines  have  a  serous 
coat  of  smooth  connective  tissue,  the  peritoneum,  which  is  continuous 
with  that  lining  the  abdominal  cavity.  From  the  serous  coat  of  the  stomach 
folds  of  peritoneum  called  omenta  (singular  omentum)  pass  to  the  large 
intestine,  to  the  liver,  to  the  spleen,  connecting  the  stomach  with  these 


98  PSYCHOBIOLOGY 

organs.  Portions  of  the  jejunal  and  ileac  divisions  of  the  small  intestine 
are  united  to  the  abdominal  wall  by  peritoneal  folds  called  mesenteries 
v.-hich  convey  the  nerves  and  blood  vessels  to  the  intestines. 

GLANDS  OF   THE   ALIMENTARY   CANAL. 

The  principal  glands  opening  into  the  human  mouth-cavity  are  the  three 
pairs  of  salivary  glands,  viz.,  the  parotid,   the  submaxillary   and  the 

sublingual  glands.  [Fig.  75.]  These  are  compound  alveolar  in  struc- 
ture. Sometimes  there  are  vestiges  of  a  fourth  pair,  the  retrolingual, 
which  are  fully  developed  in  the  dog,  cat  and  pig.  In  addition  the  mucous 
membrane  of  the  mouth,  especially  of  the  under  side  of  the  tongue,  is  full 
of  small  glands,  mainly  of  the  compound  alveolar  type.  The  mingled 
secretion  of  all  of  these  glands  is  the  saliva  which  normally  contains  des- 
quamated epithelial  cells,  disintegrating  leucocytes  and  gland  cells,  as 
well  as  inorganic  salts  and  clots  of  mucus.  The  most  important  con- 
stituent of  saliva  is  diastase :  an  enzyme  which  converts  starch  into  sugar. 
The  salivary  digestive  process  begins  in  the  mouth,  and  if  the  food  be  well 
mixed  with  saliva,  continues  for  some  time  in  the  stomach,  until  stopped 
bj  the  gastric  juice,  which  penetrates  slowly  into  the  food-lump  formed 
by  the  act  of  swallowing. 

The  secretory  cells  in  these  glands  are  of  two  types :  mucous  cells  simi- 
lar to  the  goblet  cells  already  described,  and  serous  cells,  secreting  a  more 
watery  substance.  The  secretory  cells  of  the  parotid  are  practically  all  of 
the  serous  type.  The  other  glands  are  mixed,  containing  both  serous  and 
mucous  cells.  In  the  mucous  membrane  of  the  esophagus  there  are  small 
glands  similar  in  form  to  those  of  the  oral  cavity  but  of  the  pure  mucous 
type. 

The  nerve  fibers  of  the  salivary  glands  come  from  both  the  vagus  and 
the  sympathetic  division  of  the  autonomic  system.  [The  course  of  the  sali- 
vary nerve  fibers  issuing  from  the  vagus  is  complicated ;  probably  all  issue 
in  the  nervus  intermedins.  The  fibers  to  the  parotid  glands  pass  by  the 
glossopharyngeal  nerve  to  the  Vidian  nerve  and  to  the  otic  ganglion ; 
from  thence  fibers  run  by  a  branch  of  the  fifth  nerve  to  the  gland.  The 
fibers  destined  to  the  sublingual  and  the  submaxillary  glands  run  in  the 
facial  nerve  to  the  lingual  through  the  chorda  tympani,  and  end  on  gang- 
lion cells  near  the  glands,  from  which  fibers  run  to  the  secretory  cells. 
The  sympathetic  fibers  issue  from  the  three  upper  dorsal  nerve-roots, 
pass  through  the  stellate  ganglion,  and  are  relayed  in  the  superior  cervical 
ganglion.  From  here  the  fibers  follow  branches  of  the  external  carotid 
arteries  to  the  glands.] 

The   vagal   nerve   fibers   excite   the    secretory   cells    directly    and    also 


Glands 


99 


cause  dilation  of  the  arterioles  in  the  glands  and  hence  increased 
blood  supply.  The  effects  of  currents  in  the  sympathetic  fibers  are 
not  clearly  marked,  but  include  vasoconstriction.  Apparently  the  activity 
of   the   salivary   glands   is    controlled   entirely  by   nerve   action.      Secre- 


Stenson's  duct 
Orifice  of  duct 
Parotid  gland 

Masseter  (cut) 

Mucous  membrane 

(cut)  — 

Deep  process  of sj| 

submaxillary  gland 

Mylohyoid  muscle  ___ Isp 

(cut) 
Submaxillary  gland — 

Lower  border  of  _____ — - — _3 
mandible 

Mylohyoid  muscle 

Anterior  belly  of 

digastric 
Hyoid  bone---^' 


i  Duct  of  Bartholin  (rare) 
Wharton's  duct 
Duct  of  sublingual  gland 
Sublingual  gland 


Fig.  75.  The  Salivary  glands  and  their  ducts.  (After  Cunningham.)  The  greater 
portion  of  the  lower  jaw  and  part  of  the  masseter  muscle  have  been  removed  to  show 
the  sublingual  gland  and  the  lower  part  of  the  submaxillary  gland.  Four  ducts  of 
the  sublingual  gland  are  shown,  opening  on  the  floor  of  the  mouth,  and  a  fifth  (duct 
of  Bartholin)  opening  into  Wharton's  duct.  Wharton's  duct  and  Stenson's  duct  are 
the  drains  of  the  submaxillary  and  parotid  glands  respectively. 


tion  is  normally  started  by  the  tact  and  taste  of  food  within  the  mouth 
and  by  the  smell  and  sight  of  food.  Reflex  habits  may  easily  be  built  on 
arbitrary  stimuli,  such  as  sounds.  The  ringing  of  a  bell  or  the  sounding 
of  a  tuning  fork  or  the  sight  of  a  placard  may  be  a  salivary  excitant  for 
a  dog  as  well  as  for  a  man. 


100 


PSYCHOBIOLOGY 


Salivary  reflexes  may  be  studied  in  the  human  subject  by  inserting  a 
cannula  15  in  the  duct  orifice  of  the  submaxillary  or  parotid  glands,  thus 
collecting  saliva  so  that  its  quantity  and  constitution  as  well  as  the  time  of 
its  appearance  may  be  noted.  In  work  on  animals  which  has  been  carried 
on  extensively  by  the  Russian  Pavloff 16  and  his  students,  De  Graff's 
method  has  been  followed.     De  Graff's  method  consists  in  transplanting 


Serous  gland  cells. 

Intercalated  duct. 

Mucous 


Connective  tissue 


Secretory  duct. 


Fig.  76.  Section  of  submaxillary  gland  of  an  adult  man.  Magnified  252  diam- 
eters.    (Lewis  and  Stohr,  Histology.) 

the  orifice  of  one  of  the  salivary  ducts  to  the  outside  of  the  face,  thus  mak- 
ing a  salivary  fistula  so  that  the  secretion  can  readily  be  collected  with 
minimal  discomfort  to  the  animal. 

The  glands  of  the  stomach,  which  secrete  gastric  juice,  are  tubular,  some 
being  simple,  but  the  majority  compound.  There  are  three  types  of  these 
glands:  the  fundus,   the  cardiac   and  the  pyloric   glands.     The  fundus 

15  The  cannula  is  a  tube  of  metal,  rubber,  or  some  other  hard  substance. 

16  Pavloff's  name  is  frequently  and  inconsistently  spelled  by  English  writers  after 
the  German  fashion,  Pawlow.  The  inconsistency  lies  in  using  the  German  instead  of 
the  English  transliteration  of  the  Russian  name.  As  we  can  not  conveniently  use  the 
Russian  spelling,  we  should  use,  in  English,  the  English  spelling.  The  French  ren- 
dering is  Pavlov. 


Glands  101 

glands,  which  occur  throughout  the  greater  part  of  the  stomach,  are  simple, 
cr  else  have  few  branches.  The  pyloric  glands,  which  are  larger  than  the 
fundus  glands,  occur  in  the  part  (about  one-fifth)  of  the  stomach  nearest 
the  pylorus.  The  cardiac  glands  are  situated  only  in  a  narrow  ring  near 
the  esophageal  orifice :  they  resemble  the  fundus  glands  in  size  but  are  com- 
plex like  the  pyloric  glands.  In  these  glands  there  are  two  kinds  of  cells, 
chief  cells ,  secreting  the  digestive  enzymes  (pepsin  and  rennet),  and 
■parietal  cells,  secreting  hydrochloric  acid.  The  pyloric  glands  contain 
only  chief  cells ;  the  other  gastric  glands  contain  both  kinds  of  cells.  These 
glands  are  supplied  with  nerve  fibers  derived  from  both  the  sympathetic 
and  vagal  divisions  of  the  visceral  nervous  system,  the  immediate  supply 
being  from  the  solar  plexus.  There  are  also  two  local  nerve  systems  in 
the  stomach  as  well  as  in  the  walls  of  the  intestines :  the  plexus  of  Auer= 
bach  in  the  muscular  coat,  and  the  plexus  of  Meissner  in  the  submu- 
cosa.  These  plexuses  contain  numerous  ganglion  cells  and  are  possibly 
connected  with  fibers  from  the  other  parts  of  the  autonomic  system. 

The  gastric  glands  are  excited  to  activity  primarily  by  the  same  stimuli 
(visual,  olfactory,  tactual,  etc.)  which  excite  the  salivary  glands.  The 
process  of  gastric  secretion  has  therefore  usually  been  started  before  the 
food  enters  the  stomach,  although  there  is  a  latent  period  of  several  minutes 
before  the  actual  appearance  of  the  juice.  Contact  with  or  pressure  on 
the  lining  of  the  stomach  itself  has  no  effect.  By  making  a  gastric  fistula 
and  also  a  fistula  in  the  esophagus,  Pavloff  proved  that  the  secretion  could 
bt-  produced  by  the  chewing  and  swallowing  of  food,  or  even  by  the  sight 
of  it,  although  no  food  entered  the  stomach. 

Gastric  secretion  is  also  excited  by  the  presence  of  partly  digested  food 
in  the  stomach.  This  stimulation  may  be  due  to  the  action  of  substances 
in  the  food,  or  substances  (hormones)  produced  by  the  glands  near  the 
pylorus,  acting  on  the  local  nervous  systems.  The  greatest  experts  en  the 
physiology  of  the  internal  organs  (e.  g.,  Starling),  incline,  however,  to 
think  that  the  excitation  is  due  to  hormones  which  act  directly  on  the 
gland  cells. 

The  intestines  are  provided  with  glands  of  several  types.  In  the  walls 
of  both  intestines  there  are  many  simple  tubular  glands,  Lieberkuhn's 
glands;  and  in  the  upper  part  of  the  duodenum  Brunner's  glands 
(sometimes  described  as  compound-tubular,  sometimes  as  acino-tubular) , 
are  plentiful,  becoming  less  numerous  below,  and  being  entirely  absent  at 
the  lower  end  of  the  duodenum.  Whether  the  secretions  of  Brunner's 
glands  and  Lieberkuhn's  glands  in  the  small  intestines  differ  is  not  de- 
cided. In  conjunction,  these  glands  produce  intestinal  juice  {succus 
entericus) .     The  Lieberkuhn's  glands  in  the  large  intestine  produce  a  dif- 


102  PSYCHOBIOLOGY 

ferent  secretion;  consisting  principally  of  mucus  for  lubrication,  and  pos- 
sibly containing  excreted  material.  The  two  most  important  glands  of 
the  body, — the  liver  and  the  pancreas, — discharge  their  secretions  into  the 
upper  end  of  the  small  intestine.  The  glands  of  the  intestines  are  excited 
to  activity  by  pressure  on  the  intestinal  lining  and  also  by  a  hormone 
called  secretin  which,  it  is  believed,  is  formed  by  the  epithelial  cells  of 
the  upper  part  of  the  small  intestine,  under  the  influence  of  the  acid 
chyme  (product  of  stomach  digestion).  The  intestines  are  supplied  with 
nerves  from  the  sympathetic,  the  vagus  and  (in  the  case  of  the  large  intes- 
tine, at  least)  from  the  sacral  division  of  the  autonomic  system,  and  con- 
tain plexuses  of  Auerbach  and  Meissner.  There  is  not  much  information 
available,  however,  concerning  the  nervous  control  of  the  glands.  The 
glandular  response  to  pressure  is  probably  a  reflex  from  the  local  nervous 
system.  There  are  indications  that  the  fibers  from  the  vagus  have  an  in- 
hibitory effect. 

The  liver,  the  largest  gland  in  the  body,  has  a  weight  in  the  adult  in 
the  neighborhood  of  1600  grams  (three  and  a  half  pounds).  Its  structure 
is  very  complex,  consisting  of  numerous  small  masses  (lobules)  of  secre- 
tory cells  between  which  lie  the  branches  of  the  bile  canaliculi  (corres- 
ponding to  the  tubules  or  alveolae  of  an  ordinary  gland),  and  amongst 
which  run  networks  of  blood  vessels  anl  lymph  vessels.  In  its  develop- 
ment the  liver  resembles  a  compound  tubular  gland,  but  the  tubules  anas- 
tomose with  one  another,  forming  an  intricate  network,  and  thus  losing  the 
characteristic  tubular  gland  form.  Instead  of  tubes  whose  walls  are 
formed  of  numerous  secreting  cells,  the  canaliculi  are  minute  passages 
between  the  solidly  grouped  cells,  so  that  at  most  points  the  canaliculus 
wall  is  formed  by  the  surfaces  of  only  two  cells.  The  intralobular  network 
in  each  lobule  opens  externally  into  an  interlobular  bile  duct  which  in 
turn  opens  into  a  larger  duct,  these  ducts  finally  uniting  to  form  the 
hepatic  duct.  From  the  hepatic  duct,  the  bile  is  discharged  through  the 
cystic  duct  into  the  gall  bladder  between  periods  of  intestinal  digestion, 
but  during  digestion  it  passes  from  both  the  hepatic  duct  and  the  cystic 
duct  through  the  common  bile  duct  into  the  small  intestine.  The  gall 
bladder  has  a  muscular  coat  and  there  are  numerous  bundles  of  smooth 
muscle  fibers  in  the  walls  of  the  hepatic  duct  and  the  bile  ducts,  the  fibers 
being  especially  numerous  at  the  orifice  into  the  intestine. 

The  pancreas  is  a  relatively  large  gland  weighing  about  86  grams  (3 
ounces)  and  lying  behind  the  stomach.  In  form  it  is  sacculo-tubular,  i.  e.} 
the  terminal  sacs  are  elongated,  giving  a  cylindrical  shape.  In  the  human 
body  the  main  duct  of  the  pancreas  (the  duct  of  Wirsung)  and  the  com- 
mon bile  duct  empty  into  the  duodenum  through  the  same  orifice.     The 


Glands  103 

walls  of  the  pancreatic  duct  contain  smooth  muscle  fibers.    Sometimes  there 
is  a  secondary  pancreatic  duct. 

Both  the  liver  and  the  pancreas  are  supplied  with  nerve  fibers  from  the 
solar  plexus.  Most  of  the  fibers  are  sympathetic,  but  apparently  there  are 
some  derived  from  the  vagus  nerve.  Some  of  these  fibers  run  to  the  mus- 
cular coats  of  the  blood  vessels  and  the  ducts  of  the  glands,  and  some  run 
to  the  gland  cells  themselves.  The  details  of  nervous  control  of  the  liver 
and  pancreas  are  obscure.  The  liver  secretes  continuously,  but  more 
copiously  during  digestion;  the  principal  stimulus  to  the  activity  of  both 
liver  and  pancreas  has  been  shown  to  be  the  secretin  manufactured  by  the 
small  intestine,  carried  in  the  blood  to  the  glands  and  acting  directly  on 
the  gland  cells.  The  same  hormone  also  causes  contraction  of  the  gall 
bladder,  emptying  its  contents  through  the  common  bile  duct. 

GLANDS  OF  THE  SKIN. 

The  chief  skin  glands  are  the  sweat  glands  [sudoriparous  or  sudo- 
riferous glands)  and  the  sebaceous  glands  [Fig.  57].  The  former  are 
coiled  simple  tubes,  the  latter  are  alveolar. 

The  sebaceous  glands  are  usually  associated  with  hair,  the  ducts  of 
one  to  four  glands  opening  into  the  superficial  part  of  the  hair  follicle. 
On  some  part  of  the  body  (the  lips,  for  example),  the  glands  open  on  the 
surface  independently  of  the  hairs.  The  active  cells  in  these  glands  secrete 
by  forming  sebum  within  themselves  and  then  liberating  it  by  breaking 
down ;  new  cells  from  the  deeper  layer  replacing  the  dissolved  ones.  There 
are  no  nerve  fibers  supplied  to  these  glands  and  no  muscular  fibers  in  the 
glands  themselves.  The  contraction  of  the  arrector  pili  muscle  attached 
to  a  hair  follicle  (raising  the  hair  follicle  and  thus  producing  the  condition 
known  as  "goose  flesh")  compresses  the  sebaceous  gland  and  squeezes 
out  the  sebum.  These  muscles  are  controlled  by  fibers  from  the  sympa- 
thetic division  of  the  nervous  system. 

The  sweat  glands  are  under  the  direct  control  of  the  sympathetic  nerve 
fibers  which  terminate  both  on  the  secreting  cells  lining  the  tube  and  the 
smooth  muscle  fibers  which  lie  next  to  these  cells.  These  secretory  nerve 
fibers  are  derived  from  spinal  nerve  roots  from  the  second  dorsal  to  the 
third  or  fourth  lumbar.  The  normal  stimulus  for  the  sweat-reflex  is 
heat  applied  to  the  surface  of  the  body  through  external  sources,  or  an 
increase  in  the  temperature  of  the  blood  within  the  body.  Concerning 
the  mechanism  of  the  excitation  of  the  afferent  currents  which  control 
the  efferent  current  to  the  sweat  glands,  there  is  meager  information. 

The  above  list  does  not  include  all  the  duct  glands,  nor  even  all  the  im- 
portant ones,  but  is  sufficiently  extended  to  give  an  elementary  idea  of  the 
general  facts  of  duct-gland  structure  and  function. 


104  PSYCHOBIOLOGY 

THE  DUCTLESS  GLANDS. 

Internal  secretion  is  the  production  of  hormones;  substances  which 
certain  cells  discharge  into  the  blood,  and  which  are  carried  by  the  blood 
stream  to  other  cells,  upon  which  these  substances  exercise  a  specific  action. 
A  certain  hormone  (secretin)  we  have  seen  is  produced  by  epithelial  cells 
in  the  duodenum ;  others  are  probably  produced  by  cells  in  the  epithelium 
elsewhere,  and  possibly  by  muscle  cells.17  Certain  glands,  as  for  example, 
the  pancreas,  produce  both  an  external  secretion  and  an  internal  secretion : 
the  hormone  produced  by  the  pancreas  has  a  definite  effect  on  nutritive 
processes  throughout  the  body. 

There  is  one  class  of  glands  conventionally  designated  as  ductless 
glands,  which  produce  internal  secretions  only.  The  principal  members 
of  this  class  are :  the  two  adrenal  glands  {suprarenal  capsules  or  adrenal 
bodies)  lying  near  the  upper  end  of  the  kidneys;  the  thyroid  (or  thy- 
reoid) gland  (or  body)  which  partly  surrounds  the  upper  part  of  the 
trachea  (windpipe)  and  the  pharynx  [Fig.  77]  ;  the  parathyroid  (or 
parathyreoid)  glands  of  which  there  are  usually  two  pair  lying  near  the 
thyroid;  the  pituitary  body  [hypophysis)  lying  in  front  of  the  brain 
stem  [Figs.  66  and  67]  ;  the  pineal  gland  (or  body)  just  above  the  cor- 
pora quadrigemina ;  the  carotid  body,  at  the  bifurcation  of  the  carotid 
artery;  the  cocygeal  body,  in  front  of  the  tip  of  the  cocyx  (the  terminal 
vertebrae  of  the  spinal  column)  [Figs.  29,  38]  ;  and  the  thymus  [Fig. 
77]  in  the  lower  part  of  the  neck  and  upper  part  of  the  thorax.18 

The  thyroid,  the  parathyroid,  and  in  part  the  pituitary  body  have 
alveolar  structure:  i.  e.,  they  contain  lumina  or  follicles  whose  walls  are 
composed  of  secreting  cells ;  but  these  follicles  have  no  ducts.  The  other 
ductless  glands  are  rather  of  the  solid  type:  the  cells  being  bunched  in 
masses  or  columns  between  which  the  blood  vessels  and  lymphatic  vessels 
ramify.  The  mechanism  by  which  the  hormones  enter  the  blood — whether 
directly  or  through  the  lymphatic  channels — is  not  yet  definitely  known. 

All  of  these  ductless  glands  are  supplied  with  nerves  which  apparently 
are  mostly  from  the  sympathetic  system,   although  there  are  also   fibers 

17  According  to  Howell,  the  liver  cells  produce  two  internal  secretions — glycogen 
and  urea.  The  former  is  conveyed  to  and  consumed  by  muscle  cells  throughout  the 
body;  the  latter  is  excreted  by  the  kidneys.  These  substances,  according  to  Starling, 
do  not  properly  fall  in  the  class  of  hormones ;  hormones  are  strictly  substances  which 
have  a  stimulating  or  sensitizing  effect,  as  the  derivation  of  the  term  hormones  indi- 
cates. 

18  The  ovaries,  testicles,  spleen,  and  the  lymph-nodes  are  sometimes  classed  as  duct- 
less glands.  It  is  probable  that  these  bodies  produce  hormones,  but  if  so  they  are 
not  their  principal  products. 


Glands 


105 


from  the  vagus,  cervical  and  sacral  autonomic  nerves.  The  nerve  supply 
1o  the  adrenal  glands  is  so  rich  that  these  organs  have  formerly  been  sup- 
posed to  belong  to  the  sympathetic  nervous  system.  Some  of  the  nerve 
fibers  terminate  in  connection  with  the  blood  vessels,  and  some  in  connec- 


J^^ 


Fig.  7J.  The  thyroid  and  thymus  glands  in  a  child  of  six  months.  (Schiifer. 
Microscopic  Anatomy,  after  Sappey.)  A.  The  positions  of  the  thyroid  and  thymus 
glands :  I,  2,  and  3,  right  and  left  lobes  and  median  fissure  of  the  thymus ;  6,  thyroid ; 
g,  common  carotid  artery;  10,  internal  jugular  vein;  5>  7>  and  8,  veins.  B.  Right  lobe 
of  thymus,  with  envelope  removed.  C.  The  lobe,  unravelled,  showing  the  strand  of 
connective  tissue  along  which  the  lobules  are  grouped. 


tion  with  the  secreting  cells.     Whether  there  are  any  sensory  fibers  is  not 
known. 

The  secretion  of  the  thyroid  gland  has  an  important  influence  on  the 
growth  of  all  the  bodily  tissues.  In  cases  of  removal  of  the  thyroid  gland 
a  condition  known  as  myxedema  ensues;  the  metabolic  processes  proceed 
slowly;  growth,  except  of  connective  tissue,  stops;  and  death  may  follow. 


106  PSYCHOBIOLOGY 

In  children,  atrophy  of  the  thyroid  gland  produces  the  state  of  arrested 
development  known  as  cretinism.  Cases  of  cretinism  and  myxedema  may 
be  relieved  by  feeding  the  patient  fresh  or  dried  thyroid  glands  of  animals. 
The  hypertrophy,  or  excessive  growth  of  the  thyroid,  known  as  goiter,  is 
productive  of  nervous  irritability  and  muscular  weakness.  The  function 
cf  the  parathyroids  is  somewhat  in  dispute.  In  the  view  of  some  experi- 
menters they  are  similar  in  nature  to  the  thyroid,  and  if  the  latter  be  re- 
moved without  injuring  the  former  they  can  to  a  certain  extent  fulfill  the 
the  latter's  function.     This  theory  is  probably  wrong. 

The  pituitary  body  consists  of  two  lobes,  anterior  and  posterior,  with 
a  pars  intermedia  between.  The  posterior  lobe  seems  to  have  no  secretory 
function.  The  secretion  of  the  anterior  lobe  seems  to  promote  growth, 
especially  of  the  bones  and  connective  tissue.  The  condition  known  as 
acromegaly  or  gigantism,  in  which  the  bodily  frame  grows  to  excessive 
size,  are  thought  to  be  due  to  over-development  or  over-activity  of  this 
lobe.  The  secretion  of  the  pars  intermedia  has  an  exciting  effect  on 
smooth  muscle  and  on  the  gland-cells  of  the  kidneys.  Removal  of  the 
entire  pituitary  body  causes  death. 

Of  the  functions  of  the  other  ductless  glands  little  is  known.  The 
thymus,  which  enlarges  during  the  first  two  years  of  life  and  then  di- 
minishes so  that  at  puberty  it  is  insignificant,  probably  has  a  specific  influ- 
ence on  the  growth  of  the  child.  The  pineal  body  is  glandular  only 
during  childhood,  becoming  a  mere  fibrous  body  at  adolescence.  Its  secre- 
tion probably  retards  the  development  of  the  body,  especially  the  develop- 
ment of  the  reproductive  organs.  The  secretion  of  the  adrenal  glands 
(adrenalin,  suprarenalin  or  epinephrin}  has  a  marked  effect  on  smooth 
muscle  and  gland  cells,  producing  the  same  activity  in  these  organs  as  is 
produced  by  stimulating  the  nerves  supplying  them.  Removal  of  the  ad- 
renal bodies  always  causes  death  in  from  twelve  to  twenty-four  hours. 

Cannon  has  shown  that  the  secretion  of  adrenalin  is  increased  in  ani- 
mals under  the  influence  of  stimulations  producing  such  emotions  as  fear 
or  rage.  The  importance  of  the  secretion  in  such  circumstances  can  readily 
be  understood,  since  it  acts  as  a  stimulant  to  the  muscles  and  other  organs 
and  in  particular  increases  the  liberation  of  glycogen  from  the  liver  into 
the  blood,  and  thus  increases  the  energy-supply  to  the  muscles.  The  effect 
of  adrenalin  on  the  digestive  process  is  marked.  It  checks  both  the  secre- 
tion of  the  digestive  juices  and  also  the  muscular  activity  of  the  alimentary 
canal.  It  has  long  been  known  that  certain  strong  emotions,  especially 
fear  and  rage,  are  accompanied  or  followed  by  important  changes  in  the 
digestive  process,  and  the  study  of  adrenalin  is  now  revealing  a  part  of 
the  mechanism  of  the  occurrences.     On  the  physiological  side,  the  research 


Glands  107 

■of  Cannon  and  his  pupils  opens  up  what  is  probably  the  most  important 
line  of  attack  on  the  problems  of  the  emotions. 

The  foregoing  sketch  of  the  functions  of  the  ductless  glands  should  be 
taken  as  a  statement  of  probabilities.  There  is  serious  conflict  of  opinion 
and  conflict  of  apparent  experimental  results  concerning  these  organs.  At 
the  present  time  an  enormous  amount  of  work  is  being  done  in  this  field 
by  the  physiologists,  and  the  results  of  their  researches  will  some  time  be 
cf  great  value  to  psychology. 

The  psychological  importance  of  the  ductless  glands  does  not  lie  simply 
in  the  fact  that  they  are  essential  to  the  growth,  nutrition  and  irritability 
•of  the  muscular,  glandular  and  nervous  tissues,  but  in  the  connection  which 
seems  to  exist  between  internal  secretion  and  affective  content  of  conscious- 
ress.  Although  we  have  as  yet  no  data  indicating  clearly  whether  the 
hormones  have  a  neural  stimulatory  value,  exciting  end-organs  of  affer- 
ent fibers  in  the  viscera,  or  whether  the  consciousness  factor  is  associated 
merely  with  the  nerve  reflexes  which  terminate  in  the  activation  of  the 
glands,  it  is  probable  that  the  physiological  basis  of  affective  content  will 
Li*  found  to  be  partly  in  some  phase  or  phases  of  secretion. 

The  physiological  basis  of  feeling  is  doubtless  wider  than  internal  secre- 
tion. On  the  one  hand  the  nerve  fibers  activating  the  duct-glands  are  at 
least  as  important  as  those  activating  the  ductless  glands.  On  the  other 
hand,  if  there  are  nerve  receptors  which  are  stimulated  by  hormones  there 
are  also  probably  receptors  stimulated  by  other  bodily  products,  such  as 
carbon  dioxid,  lactic  acid,  and  glycogen.  Moreover,  the  activity  of  smooth 
.muscle  (aside  from  the  smooth  muscle  involved  in  certain  of  the  glands) 
probably  plays  some  part  in  the  physiological  conditioning  of  affective 
contents  and  consciousness. 

REFERENCES    ON    GLANDS. 
I.    ON    GLANDS    IN    GENERAL    AND    DUCT    GLANDS    IN    PARTICULAR. 

Luciani,  Human  Physiology.  (Translated  by  Welby.)  Vol.  III.  (Internal  secre- 
tion, digestion,  and  excretion.) 

Birmingham,  The  Digestive  System.  Cunningham's  Anatomy.  In  particular,  § 
Glands. 

Howden,  The  Organs  of  Sense  and  the  Integument.  Cunningham's  Anatomy,  § 
The  Skin. 

Uailey,  Histology.     Chapters  V,  VI,  and  X. 

Lewis  &  Stohr,  Histology.  Topics  under  Epithelium,  The  Entodermal  Tract,  and 
Skin. 

Schafer,  Microscopic  Anatomy.     Externally  Secreting  Glands. 

Starling,  Physiology.     Chapters  X  and  XVIII. 

Jlowell,  Physiology.     Chapters  XLI-XLV. 


108  PSYCHOBIOLOGY 

Yerkes  and  Morgulis,  The  Method  of  Pawlow  in  Animal  Psychology,  Psychological 

Bulletin,   1909,  Vol.  VI,  pp.  257-273. 
Pavlov,  The  Work  of  the  Digestive  Glands  (translated  by  Thompson). 

II.   ON    THE    DUCTLESS    GLANDS. 

Cunningham,  Anatomy.    The  Ductless  Glands. 

Schafer,  Microscopic  Anatomy.     Internally  Secreting  Glands. 

Lewis  &  Stohr,  Histology.  Topics  under  Entodermal  Tract,  Suprarenal  Glands  and 
Central  Nervous  System. 

Bailey,  Histology.     Chapter  XI. 

Howell,  Physiology.     Chapter  XLVI. 

Cannon,  Recent  Studies  of  Bodily  Effects  of  Fear,  Rage,  and  Pain,  Journal  of  Phil- 
osophy, etc.   (New  York),  1914,  Vol.  XI,  pp.  162-165. 


CHAPTER  IX. 

THE   FUNCTIONAL   INTERRELATION   OF   RECEPTORS,   NEURONS,  AND 

EFFECTORS. 

The  human  body  is  a  mechanism  for  producing  response  to  stimula- 
tion. Response  is  always  the  modification  of  the  activity  of  muscles  or 
glands,  or  of  both.  Other  activities  in  the  body  (such  as  the  activities  of 
blood  corpuscles)  are  subsidiary  to  these  responses,  modifying  their  char- 
acter, temporal  course,  or  extent. 

The  responses  of  a  muscle  or  gland  are  of  two  kinds :  increase  in  activ- 
ity, or  decrease  in  activity.  In  the  muscle  the  increase  in  activity  is  mani- 
fested in  contraction;  the  decrease,  in  relaxation.  In  the  gland,  the 
activity  which  is  subject  to  increase  and  decrease  is  secretion,  that  is,  the 
formation,  or  separation,  of  some  substance  or  substances  (saliva,  mucous, 
sweat,  adrenalin,  etc.)  to  the  elaboration  of  which  the  gland  is  especially 
adapted. 

Striped  muscle  contracts  normally  only  when  irritated  by  a  nerve  cur- 
rent (excitatory  or  acceleratory  current).  Relaxation  supervenes  on  con- 
traction when  the  exciting  current  ceases,  but  is  also  facilitated  by  nerve 
currents.  These  latter  currents  are  called  inhibitory.  Whether  there  are 
two  specifically  different  classes  of  efferent  neurons,  one  class  having  ac- 
celeratory effects,  and  the  other  inhibitory  effects ;  or  whether  two  branches 
of  the  same  axon  may  have  contrary  effects  (on  two  different  groups  of 
muscle  fibers),  has  not  been  made  clear. 

In  distinction  from  the  production  of  positive  contractions,  acceleratory 
nerve  impulses  to  muscles  may  have  a  tonic  effect  (may  give  tone  to  the 
muscles) .  Tone  in  a  muscle  is  a  condition  of  preparedness  for  contrac- 
tion, and  may  be  called  the  normal  condition  of  muscle.  If  the  efferent 
nerves  supplying  a  muscle  are  completely  severed,  the  muscle  becomes 
flabby,  and  much  greater  stimulus  (e.  g.,  electricity  applied  directly  to 
the  muscle  or  to  the  cut  end  of  the  nerve)  is  required  to  produce  contrac- 
tion than  is  required  in  the  case  of  muscle  having  tone. 

The  unstriped  and  cardiac  muscle  also  receives  tone  through  nerve  cur- 
rents. In  the  normal  body,  therefore,  there  is  a  constant  flow  of  accel- 
eratory current  through  the  efferent  nerve  fibers  to  the  general  muscu- 
lature, keeping  it  in  condition  for  action. 


110  PSYCHOBIOLOGY 

Chemical  substances  carried  by  the  blood  to  muscles  are  also  an  im- 
portant factor  in  the  maintenance  of  tone.  An  increase,  for  example,  in 
the  quantity  of  adrenalin,  secreted  into  the  blood  from  the  supra-renal 
glands,  heightens  the  general  muscular  tone.  This  effect  is  not  considered, 
however,  as  a  direct  tonic  effect,  but  as  a  sensitizing  of  the  muscle,  so  that 
the  nerve  currents  produce  relatively  greater  effect. 

Smooth  muscle,  whether  in  the  intestines,  blood  vessels,  skin,  or  else- 
where is  in  general  subject  to  the  same  laws  of  stimulation  as  is  striped 
muscle.  It  seems,  however,  to  be  excited  by  other  than  nerve  action,  and 
in  particular,  contraction  of  certain  fibers  may  stimulate  adjacent  fibers. 
There  remains,  however,  some  uncertainty  on  this  point,  as  also  on  the 
points  concerning  the  causation  of  the  rhythmic  contraction  and  relaxation 
of  cardiac  muscle. 

The  acceleration  and  inhibition  of  glandular  activity  is  brought  about 
by  nervous  activity  both  directly  and  indirectly.  Directly,  the  nerve  cur- 
rents conveyed  to  the  gland  cells  seem  to  stimulate  them  to  greater  activity, 
cr  to  check  their  activity.19  Indirectly,  currents  to  the  muscular  coats  of 
the  arteries  supplying  the  glands,  by  dilating,  and  contracting  the  artejies 
and  therefore  increasing  or  decreasing  the  blood  supply,  increases  or  de- 
creases the  glandular  activity,  since  the  material  for  the  secretion  is  de- 
rived from  the  blood. 

The  important  bodily  activity,  in  short,  is  muscle  and  gland  action,  and 
this  is  to  a  large  extent  directed  by  the  nervous  system.  The  nervous  sys- 
tem, on  the  other  hand,  is  controlled  by  physical  and  chemical  stimuli 
from  objects  external  to  it.  Efferent  currents  control  the  effectors,  but 
efferent  currents  are  but  the  sequelae  of  afferent  currents  from  the  var- 
ious receptors.  The  .receptors,  in  turn,  are  excited  to  function  by  the 
action  of:  1.  Pressure  on  the  receptor  (certain  receptors  in  skin,  mucous 
membrane,  and  other  tissues).  2.  Light  (retinal  receptors).  3.  Sound 
(cochlear  ending).  4.  Substances  in  solution  (taste  bud  receptor  and  re- 
ceptors in  the  alimentary  canal).  5.  Gaseous  substances  (olfactory  cells). 
6.  Temperature  changes  (receptors  undiscovered).  7.  Muscular  contrac- 
iton  (muscle  spindle  receptors  and  receptors  in  smooth  muscle).  The 
last  form  of  stimulation  is  not  identical  with  pressure,  since  the  receptors 
in  smooth  muscle  cannot  be  stimulated  by  pressing  upon,  pinching,  or 
otherwise  maltreating  the  tissue;  but  respond  only  to  the  contraction  of 
the  tissue. 

19  Contraction  of  the  muscle  fibers  in  the  ducts  of  a  gland  may  play  an  important 
part  in  the  pouring-out  of  the  secretion.  Thus,  when  an  animal  smells  or  sees  food 
the  saliva  appears,  through  contraction  of  the  ducts,  before  there  is  a  significant  in- 
crease in  the  secretion. 


Receptors,  Neurons,  and  Effectors  111 

The  human  body  is  therefore,  physiologically  considered,  but  an  ex- 
ceedingly complicated  machine,  played  upon  by  a  great  many  external 
forces,  and  responding  to  these  forces  in  such  a  way  as  to  maintain  its 
integrity  for  a  considerable  time  and  to  produce  other  machines  similar  to 
itself.  The  physiological  performance  of  such  a  machine  may  be  summed  up 
under  two  heads.  1.  Reflexes:  processes  initiated  by  an  external  stimulus 
to  a  receptor  and  ending  in  modification  of  muscular  and  glandular 
activity.  2.  Processes  contributory  to  the  reflexes.  Under  this  last  head 
go  nutritive  and  similar  chemical  activities,  and  the  activity  of  cells  essen- 
tial to  the  nutrition  and  protection  of  the  reflex  mechanism. 

Physiology,  however,  does  not  exhaust  our  interest  in  the  organism.  It 
is  true,  so  far  as  the  organ  alone  is  concerned,  that  when  light  strikes  the 
eye,  all  that  happens  as  a  result  thereof  is  the  contraction  of  certain 
muscles,  relaxation  of  certain  others,  acceleration  of  certain  glandular 
activity  and  inhibition  of  certain  others.  But  another  thing  also  happens 
which  is  not  to  be  found  in  the  organism  at  all.  If  the  eye  be  my  eye,  I 
see  the  light  when  this  organic  reflex  occurs.  Similarly  I  am  aware  of 
the  sound  which  stimulates  my  ear,  of  the  sweet  which  stimulates  the  taste 
buds ;  and  of  the  contraction  of  the  biceps  which  stimulates  the  spindles 
therein.  Moreover,  I  am  aware  of  the  activities  of  my  viscera  through 
the  operation  of  the  reflexes  in  which  these  take  part ;  an  awareness  which 
(in  contradiction  to  the  awareness  of  these  operations  which  I  might  have 
through  visual  reflexes,  for  example,  if  my  viscera  were  laid  open  for 
microscopic  examination)    I  call  'having  feelings'. 

These  awarenesses  (which  together  we  call  consciousness)  depend 
upon  the  action  of  reflexes.  Without  a  reflex  from  the  eye  I  cannot  see 
light.  Without  reflexes  from  my  viscera  I  cannot  have  '  feelings  '.  To 
say  that  the  reflexes  cause  the  consciousness  is  to  make  an  extrascientific 
assumption  which  is  not  justified,  unless  we  mean  by  '  cause  '  no  more  than 
invariably  accompany. 

The  awarenesses  just  indicated  are  perceptual.  In  addition  there  is  a 
form  of  consciousness  which  we  call  thought.  I  can  think  of  red  light, 
when  the  appropriate  stimulus  does  not  fall  on  the  eye,  and  when  there- 
fore the  perceptual  light  reflex  does  not  occur.  There  is,  in  this  case, 
doubtless  a  reflex,  which,  although  not  initiated  in  the  same  receptor  as 
the  perceptual  light  reflex,  has  the  same  termini  as  the  latter.  The  initia- 
tion as  well  as  the  termination  of  one  of  these  thought-reflexes  is  probably 
always  the  contraction  of  striped  muscle.  Thus,  on  this  assumption,  all 
forms  of  consciousness  are  concomitants  of  reflexes. 

The  important  question  now  is :  what  sort  of  reflexes  condition  con- 
sciousness ?     The  answer  is :  those  which  take  place  through  the  '  central 


112  PSYCHOBIOLOGY 

nervous  system'  (brain  and  cord).  The  reflexes  through  local  ganglia 
(such  as  the  ganglia  in  the  walls  of  the  alimentary  canal,  or  in  the  heart), 
are  to  be  excluded,  as  probably  having  no  part  in  the  conditioning  of  con- 
sciousness. 

It  is  commonly  supposed  that  only  the  reflexes  which  take  place  through 
the  cortex  are  '  conscious  '.  In  fact,  it  is  often  held  that  it  is  the  action  of 
cortical  cells  which  is  the  ultimate  condition  of  the  consciousness.  For 
this  view  there  seems  to  be  no  strong  evidence.  For  aught  we  know  action 
of  muscle  may  be  the  more  essential  part  of  the  process,  and  it  is  safest 
for  the  present  to  make  no  attempt  at  localization  within  the  total  reaction 
process.  We  cannot  even  admit  that  it  is  essential  for  the  production  of 
consciousness  that  the  arc  (or  path)  of  the  reflex  should  lead  through  the 
cortex.  A  spinal  reflex  has  all  the  essential  conditions  of  consciousness, 
so  far  as  we  know.  It  would  be  rashly  dogmatic  even  to  say  positively 
that  the  reflexes  within  the  intestinal  plexuses  of  Auerbach  and  Meissner 
do  not  condition  consciousness. 

In  order  to  avoid  confusion,  the  reader  must  here  note  that  the  term 
reflex  is  here  used  in  the  strict  technical  sense  to  designate  the  total  process 
taking  place  over  an  arc;  that  is,  the  process  beginning  in  an  end-organ 
(such  as  the  cone  in  the  retina),  passing  along  a  series  of  neurons  (arc) 
to  the  spinal  cord,  and  in  some  cases  from  thence  to  the  brain,  and  finally 
reaching  some  effector  (muscle  or  gland  cell),  or  effectors,  whose  activity 
is  consequently  modified.  This  process  is  called  a  '  reflex  '  because  it  may 
be  thought  of  (in  an  untechnical  way)  as  a  process  which  is  directed  in- 
ward to  the  nerve  center,  and  from  thence  reflected  out  again  to  the 
periphery. 

Unfortunately  the  process  in  which  a  reflex  terminates,  which  is  properly 
called  reflex=action,  has  come  to  be  described  by  many  writers  by  the 
shorter  name  '  reflex  '.  This  is  bad  usage.  The  mere  contraction  of  the 
iris,  for  example,  when  light  is  suddenly  thrown  into  the  eye,  is  a  '  reflex 
contraction  '  but  is  not  a  '  reflex  '.  The  '  reflex  '  is  the  total  process  begin- 
ning with  the  retinal  activity  produced  by  the  light  stimulus  and  terminat- 
ing in  the  pupillary  contraction. 

Another  source  of  confusion  lies  in  the  fact  that  formerly  it  was  assumed 
that  only  a  limited  class  of  activities  are  the  results  of  reflexes.  '  Reflex 
action  '  was  set  over  against  '  voluntary  action  ',  and  sometimes  other  forms 
of  action  were  distinguished  from  these.  The  more  modern  view,  which  is 
adopted  here,  considers  all  normal  actions  as  the  termini  of  reflexes:  it 
holds,  for  example,  that  voluntary  actions,  such  as  dropping  a  letter  in  the 
box  after  deliberating  whether  to  post  it  or  not,  are  just  as  much  '  reflex 
actions '  as  are  the  blinking  of  the  eye  when  a  cinder  strikes  it,  and  the 
deep  inhalation  which  follows  the  perception  of  a  faint  pleasant  odor. 


Receptors,  Neurons,  and  Effectors  113 

THE   FUNCTIONAL  UNITY  OF   THE   CENTRAL   NERVOUS   SYSTEM. 

Before  considering  further  the  dependence  of  consciousness  on  organic 
reflexes,  it  is  necessary  to  look  at  the  nervous  system  from  the  point  of 
view  of  its  mode  of  function. 

An  afferent  impulse  over  a  single  neuron  is  capable  of  being  trans- 
mitted to  any  efferent  neuron  of  the  centralized  nervous  system,  including 
the  visceral  division  (but  excluding  of  course  the  local  system:  plexuses 
of  Auerbach  and  Meissner),  or  to  a  large  number  of  such  neurons.  That 
is  to  say:. the  irritation  of  an  afferent  neuron  may,  through  the  successive 
passing  of  the  irritation  to  various  intermediate  (associative  and  com- 
missural) neurons  cause  the  irritation  of  any  efferent  neuron,  or  of  a  num- 
ber of  such  neurons,  and  so  may  modify  the  activity  of  any  muscle  or 
gland,  or  of  a  large  number  of  effectors. 

Impulses  are  constantly  passing  inward  over  the  afferent  chains.  Even 
in  sleep,  the  only  afferent  neurons  which  suspend  their  activity  are  those 
from  the  retina,  and  from  some  of  the  striped  muscles,  and  possibly  from 
some  small  areas  of  the  skin.  The  afferent  terminals  in  smooth  muscle, 
and  in  the  muscles  of  the  breathing  mechanism,  are  still  being  stimulated, 
with  about  normal  response.  Conversely,  there  is  a  continuous  and  widely 
distributed  outgo,  maintaining  the  tone  of  the  muscles,  and  stimulating  or 
inhibiting  contraction  and  secretion.  Even  in  sleep,  the  control  of  the 
visceral  organs,  and  of  respiration,  cannot  be  suspended,  and  the  tone  of 
the  striped  muscle  generally  must  be  maintained. 

The  neural  mechanism,  therefore,  must  not  be  regarded  as  a  collection 
of  potential  or  actual  arcs,  but  as  one  enormously  complicated  arc,  in 
which,  for  legitimate  purposes  of  description  we  distinguish  multitudinous 
paths  from  sensory  periphery  to  motor  periphery.  These  individual  arcs 
are  not  fictitious,  but  are  to  a  certain  extent  abstractions. 

The  following  example  of  a  reflex  may  make  the  relation  of  total  sys- 
tem and  particular  function  more  clear.  The  organism  may  be  so  dis- 
posed that  a  stimulus  to  the  eye  produces  a  specific  movement  of  the  hand ; 
this  is  the  case  in  a  simple  reaction  measurement  when  the  reactor  is  in- 
structed to  press  a  rubber  bulb  immediately  on  seeing  a  light.  In  this 
case  there  is  probably  a  discharge  through  a  series  of  neurons  running 
from  the  retina  of  the  eye  through  the  mid-brain  (and  possibly  through 
the  cerebral  cortex),  down  the  spinal  cord,  and  out  through  the  spinal 
root  to  the  muscle.  The  afferent  current  from  the  visual  mechanism  is 
not.  however,  distributed  solely  to  the  muscular  apparatus  involved  di- 
rectly in  the  production  of  the  specific  reaction  prescribed  in  the  instruc- 
tions. The  irritation  spreads  to  other  neurons,  chains  going  to  other 
effectors,  as,  for  example,  to  the  extrinsic  and  intrinsic  muscles  of  the  eye 


114  PSYCHOBIOLOGY 

itself,  to  the  heart,  etc.  The  optic  tract  neurons  from  retina  to  mid-brain, 
in  other  words,  are  the  common  beginning  of  a  great  many  diverging  arcs. 
Conversely  the  discharge  to  the  arm  is  not  derived  solely  from  the  visual 
apparatus,  but  is  derived  from  a  number  of  sources  not  definitely  anal- 
yzed ;  the  efferent  neurons  in  this  chain,  that  is,  are  simultaneously  excited 
from  many  different  directions,  and  one  such  chain  represents  or  com- 
bines in  itself  the  efferent  terminal  divisions  of  a  large  number  of  arcs. 
It  is  the  action  of  these  arcs  other  than  the  dominating  one — other  than 
the  arc  from  eye  to  arm  muscle — which  brings  about  by  the  preliminary 
arcs  caused  by  the  instructions  and  corresponding  to  the  intention  to  react, 
the  formation  of  the  dominant  reflex-arc. 

REFLEX  DOMINANCE. 

We  are  now  in  a  position  to  consider  the  question  of  consciousness 
from  the  point  of  view  of  the  dominance  of  reflexes.  The  retinal  arc  in 
the  case  above  described  may  be  said  to  dominate  the  total  system  in  the 
sense  that  for  the  time  it  is  the  central  line  of  discharge,  in  regard  to 
which  all  other  lines  are  derivatives  or  contributory.  All  the  afferent 
channels  are,  for  the  moment,  secondary  in  their  effects,  and  all  the  efferent 
channels  are  subordinated  to  the  demands  of  the  dominating  one. 

The  condition  of  dominance  and  subordination  is  probably  typical  of 
the  reflexes  which  condition  perceptual  consciousness.  In  such  cases 
the  discharge  through  the  pathway  from  the  sense  organ  affected  domi- 
nates the  nervous  system.  The  visceral  discharges  are  in  general  less  af- 
fected than  the  somatic  by  such  dominance,  but  in  cases  where  there  is  a 
strong  emotional  factor,  as  when  a  fearful  or  pleasing  object  is  per- 
ceived, there  must  be  a  considerable  disturbance  of  the  visceral  efferent 
system. 

The  essential  condition  of  attentive  consciousness  seems  to  be  the  func- 
tioning of  the  nervous  system  as  a  whole.  We  have  no  reason  to  assume 
that  any  reflex  takes  place  without  consciousness  of  some  sort.  If  the 
functions  of  the  system  were  diffused — no  arc  dominating — there  might 
theoretically  still  be  consciousness,  but  it  would  be  absolutely  inattentive ; 
of  zero  vividness.  If  the  afferent,  associative,  and  efferent  neurons  con- 
stituting a  single  arc,  or  a  large  group  of  arcs,  could  be  split  off  from  the 
remaining  system,  and  still  function,  the  function  would  possibly  be  ac- 
companied by  consciousness;  two  streams  of  consciousness  might  (by  this 
hypothesis)  go  in  connection  with  the  same  individual.  Such  a  condition 
seems  to  be  found  in  certain  cases  of  hysteria.  This  is,  however,  a  matter 
which  is  open  to  different  interpretations,  and  is  not  within  the  range  of 
the  present  discussion. 


Receptors,  Neurons,  and  Effectors  115 

"  CENTERS  "  IN  THE  BRAIN  AND  CORD. 

In  dealing  with  neural  functions  many  physiologists  and  physiologizing 
psychologists  make  much  of  the  concept  of  centers.  This  concept  is  on 
the  whole  exceedingly  vague,  but  there  are  two  somewhat  definite  forms 
which  it  is  important  to  notice. 

1.  The  Phrenological  Theory  of  Centers. 

There  is  a  tendency  to  use  the  word  '  center  '  in  an  occult  way,  to  de- 
scribe certain  parts  of  the  neural  mechanism  as  if  certain  functions  of  con- 
sciousness and  of  motor  control  were  literally  located  in  particular  groups 
of  cells.  The  '  visual  center  '  is  postulated  as  the  place  in  which  vision 
takes  place.  So  the  other  sensory  centers — olfactory,  auditory,  etc. — in 
the  cortex  are  considered  as  groups  of  cells  on  the  action  of  which  depend 
directly  the  various  states  of  consciousness  (and  in  fact  the  various  con- 
tents thereof),  described  by  the  various  senses.  The  consciousness  of 
light  is  supposed,  on  this  hypothesis,  to  be  caused  by  (or  to  be  concomi- 
tant with)  the  action  of  certain  cells  in  the  occipital  lobes  of  the  cortex, 
and  of  these  cells  solely.  The  fact  that  vision  does  not  occur  without 
stimulation  of  the  retinal  endings  is  explained  as  due  to  the  impossibility 
of  properly  exciting  these  cortical-visual  cells  except  by  a  current  from 
the  rods  or  cones  of  the  retina ;  but  the  occurrence  of  vision  is  nevertheless 
supposed  to  depend  in  a  direct  and  intimate  way  on  these  cortical  cells. 

Over  against  these  sensory  centers,  there  are  supposed  to  be  a  set  of 
motor  centers  which  are  in  direct  control  of  the  various  complex  activities 
of  the  muscles.  Not  only  have  centers  of  various  groups  of  muscles  been 
described,  but  also  centers  for  the  control  of  various  groups  in  special 
ways.  Thus,  for  example,  there  has  been  supposed  to  be  a  '  writing 
center  '  which  controls  the  muscles  of  the  arm  and  hand  in  movements  of 
writing  words ;  a  control  distinct  from  that  exercised  over  the  same 
muscles  by  other  centers  for  other  purposes.  Centers  for  respiration,  and 
for  vasomotor  control  have  been  located.  These  various  centers  are  fig- 
ured as  possessing  a  sort  of  spontaneity  or  intelligence,  so  that  they 
simply  need  to  be  stimulated  from  other  centers  in  order  to  operate  the 
mechanisms  under  their  care.  It  is  not  probable  that  the  centers  possess 
any  great  degree  of  functional  independence,  and  the  view  should  be 
avoided. 

2.  The  Theory  of  the  Center  as  a  Distributing  or  Collecting  Organ. 
The  term  '  center  '  ought  by  rights  to  be  abandoned  altogether ;  but  if 

used  at  all  it  is  properly  employed  to  designate  a  nucleus,  ganglion,  or 
other  group  of  cells  from  which  impulses  may  be  sent  forward  in  several 
divergent  lines  (sensory  center)  or  into  which  impulses  are  collected  from 
several  proximal  sources  (motor  center). 


116  PSYCHOBIOLOGY 

The  afferent  neurons  of  the  spinal  system  form  chains  along  which  the 
impulse  is  passed  from  neuron  to  neuron.  Some  of  these  chains  ultimately 
reach  the  cerebral  cortex.  Some  afferent  chains  possibly  have  not  direct 
connection  with  the  cortex.  The  synaptic  connections  between  two  serially 
contiguous  neurons  occur  usually  in  regions  of  the  cord  or  brain-stem 
where  lie  the  cell-bodies  of  the  second  of  the  two  neurons.  Such  groups 
are  in  the  various  nuclei  of  the  brain-stem,  and  in  Clark's  column  of  the 
cord.  If  one  of  these  nuclei  is  a  simple  relay  station,  passing  the  impulse 
always  on  over  the  same  route,  it  could  not  be  called  a  center.  But,  if 
from  a  given  nucleus,  a  current  received  from  a  peripheral  neuron  can  be 
transmitted  either  towards  the  cortex,  or  to  a  certain  motor  nucleus  di- 
rectly, the  nucleus  in  question  is  a  center.  So  with  the  nuclei  in  the  effer- 
ent chains.  If  they  receive  from  several  different  sources,  they  are 
properly  called  motor  centers. 

The  centers  in  the  cerebral  cortex  are  called  the  higher  centers.  The 
centers  in  the  brain-stem  and  cord  are  called  lower  centers.  Each  of  the 
afferent  systems  have  one  or  more  lower  centers,  although  not  all  have 
cortical  centers.  The  connections  of  these  centers  and  the  other  nuclei 
are  not  completely  known ;  even  some  of  the  connections  which  have  been 
carefully  studied  are  in  dispute ;  and  the  known  details  are  so  complicated 
that  they  cannot  be  well  introduced  here.  It  is  sufficient  for  the  present 
purpose  to  understand  that  the  afferent  and  efferent  neurons  form  an 
enormously  complicated  system,  in  which  afferent  impulses  can  be  dis- 
tributed and  collected  through  many  synapses  in  the  brain  and  cord,  and 
hence  any  incoming  current  can  issue  into  almost  any  efferent  channel. 

REFERENCES  ON   THE  FUNCTIONAL  RELATION  OF  RECEPTOR,   NEURONS,    AND 

EFFECTORS. 

McDougall,  W.,  The  physiological  factors  of  the  attention  process.     Mind,  1902,  N.  S., 

xi,  316-351-    1903.  xii,  289-302,  473-489- 

Lewes,  The  physical  basis  of  mind. 

Starling,  Physiology,  chapter  VII. 

Howell,  Physiology,  §  II   (chapters  VI-XIII). 

Schafer,  The  cerebral  cortex.     Schafer's  Text-Book  of  Physiology,  II,  697-782. 

Sherrington,  The  spinal  cord.     Schafer's   Text-Book  of  Physiology,  II,  783-883. 


INDEX. 


Abdu'cent  nerve,  83 
Acromegaly,  106 
Acinous  glands,  95-6 
Adipose  tissue,  24 
Adrenal  glands,  104,  106 
Adrenalin,  106,  107,  109 
Afferent  nerve  endings,  57 

neurons,  55 
Alimentary  canal,  97  ff 
Al'veolar  glands.  95 

structure,  13 
Amoe'ba,  19 
Ameboid  movement,  19 
Anab'olism,  14 
An//astomo/sis,  33 
Anisotropic  substance,  30 
Anterior  roots,  70 
Aqueduct  of  Sylvius,  76-7 
Aracb/noid,  51 
Are'olar  tissue,  23 

Arrec'tor  pi'li,  arecto'res  pilo/rum,  103 
Assimilation,  19 
Auditory  nerve,  84 

nerve  endings,  58-9 
Auerbach,  plexus  of,  91-2,  102 
Autonomic,  see  Splanchnic 
Ax'on,  42 

Biedermann's  fluid,  36 
Bile  duct,  102 
Bipolar  cells,  55,  69 
Blas'tula,  21 
Blind  spot,  70 
Blood  cells,  12,  13 
Bone,  25-6 

cells,  26 
Brain,  anatomy  of,  73  ff 

embryonic,  40-1 

stem,  74 
Brunner's  glands,  96,  101 


BuFbus  olfacto'rius,  68 

Caelum  (see  'cum),  97 
Canalic'uli,  bile,  102 
Capsules  of  receptors,  60 

of  thalamus,  79 
Cardiac  glands,  100 

muscle,  32-3 
Carotid  body,  104 
Cartilage,  25-6 

cell,  26 
Cell,  structure  of,  11  ff 

division,  16,  17 

membrane,  14 
Centers,  brain,  115 

visual,  82 
Cen'trosomes,  14 
Cei^ebeFlum,  73,  77 
Cer//ebrospi/nal  fluid,  51 

system.  54,  86 
Chi'asm  (ki'asm),  optic,  70 
Chief  cells,  101 
Chromatin,  14 
Chromosomes,  17 
Chyme  (kime),  102 
Cilia,  17,  23,  60 
CircumvaFlate  papillae,  59 
Clark's  column.  57 
Coats  of  alimentary  canal,  97 
Coccygeal  (kok-sij'-e-al)  body,  104 
Coc'cyx  (kok'six),  104 
Collateral  ganglia,  91 
Colon,  97 

Columns  of  cord,  79 
Com//misur/al  neurons,  45 
Com'misures  of  brain,  79 
Compound  glands,  96 
Conduction,  neural,  43-4 
Cones,  retinal,  68-9 
Connective  tissue.  23,  25 


118 


Index 


Consciousness,  111 

Contraction  of  muscle,  35  ff. ,  109 

Co/rium,  68 

Cortex,  cerebral,  74 

Corti,  organ  of.  59 

Corpus  callo'sum,  79 

Cor'poia  striata.  74 

quad//rigem/ina,  74 
Cre'tinism,  106 
Cribiform  plate,  68 
Crus.  crura,  74,  77 
Cuboidal  cells,  22 
Current  of  demarkation,  39 

neural,  44 
Cu'neate  tu'bercules,  77 
Cystic  duct,  102 
Cytolyruph,  13 
Cytoplasm,  13 

Dendrite,  dendron,  42 

Dienceph/alon,  74 

Disc  (of  eye),  see  Blind  spot 

Duct  glands,  95 

Duode'num,  97 

Dura  mater,  51 

Ectoderm,  21 

Egg  cell,  development  of,  21 
Emotions,  106 
End  bulbs,  63 
Endoderm,  21 
End  plate,  72 

Ep^inepb/rin,  see  Adrenalin 
Epineu'rium,  49 
Epithelium,  21-3 
Esophageal  orifice,  97 
Esophagus,  97 
Excretion,  94 

Facial  nerve,  94 
Fas'cia  (fash-i-a),  29 
Fascic'uli,  nerve,  49 

of  tendon,  64 

of  cord,  79,  82 
Fatigue,  38 
Fibrils,  muscle,  29,  32 

nerve,  46 
Fission,  16 
Fissures,  of  cord,  79 


Fistula,  salivary,  99 
Foliate  papillae,  59 
Fo/vea  centralis,  68 
Free  nerve  endings,  58 
Fundus  of  gland,  96 

gland,  100 
Fungiform  papillae,  59 
Funic'uli  of  nerve,  49 
Funiculus  cuneatus,  77 

gracilis,  77 
Gall  bladder,  102 
Ganglia,  collateral,  88 

sympathetic,  88 
Ganglion  cells,  70 

Gasserian,  88 

mesenter/ic,  88 

semilunar,  88 
Genic'ulate  bodies,  70,  79 
Genital  corpuscles,  63 
Germ  layers,  20 
Gland  cells.  94 
Glands,  94  ff 

ductless,  104  ff 

importance  of,  for  psychology.  107 

intestinal,  101-2 

of  skin,  103 

of  stomach,  100-1 
Glosso-pharyn'geal  nerve,  60,  84,  90 
Goblet  cells,  23,  97 
Golgi,  organ,  62.  64 
Gray  matter,  54 
Gustatory  cell,  59.  60 

nerve  endings.  59.  60 

Hair  cells,  59 
Hairs,  olfactory.  68 
Hemispheres,  cerebral,  74.  79 
Henle.  sheath  of,  48 
Hepat/ic  duct,  102 
Hind  brain,  83 
Horizontal  cells.  69 
Hor'mones,  95.  104,  110 
Hy'aline  membrane.  67 
Hyaloplasm,  13 
Hypoglossal  nerve,  85 
Hypophysis,  77,  104,  106 

ll'eum,  97 
Infundib'ulum,  77 


Index 


119 


Insertion  (of  tendon),  31 
Intervertebral  fora'men,  49 
Intestine,  97 
Intestinal  juice,  101 
Isotropic  substance,  30 
Irritability,  36 

Jeju'num,  97 

Kar//yokine/sis,  17 
Katab/olism,  14 

Latent  period,  34 

Lateral  ganglion,  91 

Leucocyte,  13 

Lieberkiihn's  glands,  101 

Ligamen/ta  denticula/ta,  52 

Lingual  nerve,  60 

Li'nin,  14 

Lissauer's  tract,  56 

Liver,  96,  102,  103 

Local  system  (nerve),  88,  91 

Mac'ula  lu/tea,  68 
MeduFla  (oblongata),  73 
Med/ullary  groove,  40 

tube,  40 
Meibo'mian,  see  Tarsal 
Meissner's  corpuscles,  61 
Meissner,  plexus  of,  91-2,  101-2 
Membrane,  basement,  97 

hyaline,  67 

mucous,  97 

vitreous,  67 
Mesencephalon,  74 
Mesenchymal  cells,  22 
Mes'oderrn,  21 
Metabolic,  14 
Metaplasm,  14 
Mesencephalon,  73 
Micro-millimeter,  12 
Mikron,  12 
Mitosis,  16,  17 
Mitotic  spindle,  17 
Morula,  21 
Mucous  cells,  48 

tissue,  23 
Muscle,  chemistry  of,  37-8 

columns,  29 


Muscle,  electrical  properties  of,  38-9 

fibers.  29,  32 

function,  33  ff 

spindles,  65 

stimulation  of,  34 
Muscles  of  eye,  83 
My//elenceph/alon,  73 
Myelin,  48 
Myoblasts,  28 
Myxede/ma,  105 

Nerve  cells,  42 

endings,  37  ff 

fibers,  42 

sheaths,  48 

tissue,  26,  40  ff 
Nerves,  cranial,  82,  85 

spinal,  82 
Nervous  system,  functional  unity  of,  113 
Neural  crest,  40,  48 
Neu//rilem/ma,  48 
Neuroglia,  53 
Neurokeratin,  45 
Neu'ron,  42  ff 
Neuroplasm,  46 
NTode  of  Ranvier,  49 
Nuclear  juice,  14 
Nucle'olus,  14 
Nucleus  cauda/tus,  79 

of  cell,  12,  14 

lateralis,  77 

lentic//ula'ris,  79 

medians,  77 

Oculo-motor  nerve,  82 
Olfactory  bulb,  68,  79 

cells,  67 

glomei-/uli,  68 

hairs,  68 

nerve,  82 
Olives,  77 
Omen'tuni,  a.  97 
Optic  nerve,  82 
Origin  of  tendon,  31 
.Osteoblast,  25-6 
Ode  ganglion,  90 

Pacinian  corpuscles,  62-3 
Pain,  referred,  93 


120 


Index 


Pancreas  96,  102,  103 

Papillae  of  tongue,  59 

Parathyroid  glands,  104,  106 

Parietal  cells,  101 

Parotid  gland,  98 

Pathetic  nerve,  83 

Pelvic  visceral  nerves,  91 

Perceptual  consciousness,  111,  114 

Peripheral  ganglia,  89 

Perimysium,  31 

Perineurium,  44 

Peritoneum,  97 

Pharynx,  97 

Pineal  body.  77,  104,  106 

Pifuitary  body,  77,  104,  106 

Plastids,  14 

Plexuses  of  afferent  fibers,  58 

of  Auerbach  and  Meissner,  91-2,  101 . 
102 
Pneumogas/tric  nerve,  see  Vagus  nerve 
Pons,  63,  77 

Post-ganglionic  fibers,  87,  89,  90 
Posterior  columns,  56 
Pia  mater,  49 
Pore-canal,  60 

Pre-ganglionic  fibers,  87,  88,  89 
Pn/'priocep'tive  neurons,  83 
Prosencephalon,  fore-brain,  74 
Protoplasm,  12 
Pseudopo/dia,  18 
Pulvi/nar,  79 
Pylo'rus,  97 
Pyloric  gland,  100 
Pyramids,  77 

Rac'emose  glands,  96 

Ramus    communicans      (rami    communi- 

cantes),  57,  71,  88 
Receptors,  110 
Rectum,  97 
Reflex,  91 

-action,  112 

-arc,  112 
Reflexes,  99,  100,  111  ff 

axon,  46 

salivary,  99,  100 
Refractory  period,  37 
Relaxation,  109 
Resting  current,  39 


Reticular  tissue, 
Retrolingual  gland,  98 
Rhornb//enceph/alon,  73 
Rod  cells,  68 
Rods,  retinal,  68-9 
Ruffini's  nerve  endings,  64,  66 

Saccular,  see  Acinous 
Sa/cral  nerves,  88,  90 
Saliva,  98 

Salivary  glands,  96,  98 
Sarcolae'tic  acid,  38 
Sarcoleni'ma,  29 
Sar'coplasm,  29 
Schwann,  sheath  of,  48 
Seba'ceous  glands,  103 
Se'bum,  103 
Secretion,  94  ff 

internal,  104  ff 
Secretin,  102.  104 
Septum,  of  cord,  53 
Se'rous  cells,  98 
Sheath,  of  hair,  66-7 
Smooth  muscle,  31-2 

contraction  of,  37 
Somatic  fibers,  70 

Splanchnic  ganglia,  nervous  system,  89' 
Spinal  accessory  nerve,  84 

cord,  49  ff,  73 

fora'men,  49 

ganglia,  41,  44 

nerves,  49 
Spireme,  17 
Spon'gioplasrn,  13 
Squamous  cells,  22 
Stellate  ganglion,  77 
Stomach,  97 
Stra'tuni  cor'neum,  61 

gefminatfvum,  62 

hfciduin,  61 

muco/sum,  62 
Striated  muscle,  29 
Stroma,  97 
Sublingual  gland,  98 
Submaxillary  gland,  98 
Submuco'sa,  97 
Sudoriferous,  see  Sweat 
Sudoriparous,  see  Sweat 
Supporting  cells,  60,  67 


Index 


121 


Suprare'nal,  see  Adrenal 
Sustentac'ular,  see  Supporting 
Sweat  glands,  103 
Sympathetic  ganglia,  41 

nerves,  99 

system,  54,  88,  90 
Syn/apse,  synapsis,  45 
Syncytium,  22,  32 

Tactile  cells,  58 

discs,  58 
Tarsal  glands,  96 
Taste  bud,  59 

pores,  60 
T-cells,  56 
Telencephalon,  74 
Tendon,  24 

spindle,  64-5 
Tet/anus.  35 
Terminal  corpuscles,  62 

ganglia,  91 
Thalamus,  74,  77 
The'ca,  60 


Thought,  111 
Thymus  gland,  104,  106 
Thy'roid,  104  ff 
Tone  (tonus),  34-5,  109 
Tract,  olfactory,  79 
of  cord,  79,  82 
Trifacial,  see  Trigeminal 
Trigeminal  nerve,  83 
Trochlear,  see  Pathetic 
Tuber  cine'reum.  77 
Tubular  glands,  95 
Tu'nica  pn/pria,  97 

Vagus  nerve,  84,  90 

function  of,  98 
Vascular  tissue,  26 
Ventricles  of  brain,  40,  75-6 
Visceral,  See  Splanchnic 
Voluntary  action,  112 

Wandering  cells,  19 
White  matter,  53 
Wirsung,  duct  of,  102 


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